diff --git a/Cargo.lock b/Cargo.lock
index 49d10a9457..6dbc4af741 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2886,6 +2886,7 @@ dependencies = [
  "serde_json",
  "serde_yaml",
  "state_processing",
+ "store",
  "tree_hash",
  "types",
  "validator_dir",
@@ -3657,6 +3658,7 @@ dependencies = [
  "eth2_ssz",
  "itertools",
  "parking_lot",
+ "rayon",
  "serde",
  "tree_hash",
  "triomphe",
diff --git a/beacon_node/store/src/chunked_vector.rs b/beacon_node/store/src/chunked_vector.rs
index 9c248ec4f3..c0b8572d43 100644
--- a/beacon_node/store/src/chunked_vector.rs
+++ b/beacon_node/store/src/chunked_vector.rs
@@ -21,9 +21,6 @@ use tree_hash::TreeHash;
 use typenum::Unsigned;
 use types::VList;
 
-#[cfg(feature = "milhouse")]
-use types::milhouse::ImmList;
-
 /// Description of how a `BeaconState` field is updated during state processing.
 ///
 /// When storing a state, this allows us to efficiently store only those entries
@@ -301,7 +298,7 @@ field!(
     T::SlotsPerHistoricalRoot,
     DBColumn::BeaconBlockRoots,
     |_| OncePerNSlots { n: 1 },
-    |state: &BeaconState<_>, index, _| safe_modulo_index(state.block_roots(), index)
+    |state: &BeaconState<_>, index, _| safe_modulo_index_vect(state.block_roots(), index)
 );
 
 field!(
@@ -311,7 +308,7 @@ field!(
     T::SlotsPerHistoricalRoot,
     DBColumn::BeaconStateRoots,
     |_| OncePerNSlots { n: 1 },
-    |state: &BeaconState<_>, index, _| safe_modulo_index(state.state_roots(), index)
+    |state: &BeaconState<_>, index, _| safe_modulo_index_vect(state.state_roots(), index)
 );
 
 field!(
@@ -323,7 +320,7 @@ field!(
     |_| OncePerNSlots {
         n: T::SlotsPerHistoricalRoot::to_u64()
     },
-    |state: &BeaconState<_>, index, _| safe_modulo_index(state.historical_roots(), index)
+    |state: &BeaconState<_>, index, _| safe_modulo_index_list(state.historical_roots(), index)
 );
 
 field!(
@@ -333,7 +330,7 @@ field!(
     T::EpochsPerHistoricalVector,
     DBColumn::BeaconRandaoMixes,
     |_| OncePerEpoch { lag: 1 },
-    |state: &BeaconState<_>, index, _| safe_modulo_index(state.randao_mixes(), index)
+    |state: &BeaconState<_>, index, _| safe_modulo_index_vect(state.randao_mixes(), index)
 );
 
 pub fn store_updated_vector<F: Field<E>, E: EthSpec, S: KeyValueStore<E>>(
@@ -572,7 +569,25 @@ fn safe_modulo_index<T: Copy>(values: &[T], index: u64) -> Result<T, ChunkError>
 }
 
 #[cfg(feature = "milhouse")]
-fn safe_modulo_index<V: ImmList<T>, T: Copy>(values: &V, index: u64) -> Result<T, ChunkError> {
+fn safe_modulo_index_list<T: TreeHash + Copy, N: Unsigned>(
+    values: &VList<T, N>,
+    index: u64,
+) -> Result<T, ChunkError> {
+    if values.is_empty() {
+        Err(ChunkError::ZeroLengthVector)
+    } else {
+        values
+            .get(index as usize % values.len())
+            .copied()
+            .ok_or(ChunkError::OutOfBounds)
+    }
+}
+
+#[cfg(feature = "milhouse")]
+fn safe_modulo_index_vect<T: TreeHash + Copy, N: Unsigned>(
+    values: &FixedVector<T, N>,
+    index: u64,
+) -> Result<T, ChunkError> {
     if values.is_empty() {
         Err(ChunkError::ZeroLengthVector)
     } else {
diff --git a/beacon_node/store/src/partial_beacon_state.rs b/beacon_node/store/src/partial_beacon_state.rs
index aae54c4f19..160f94aef2 100644
--- a/beacon_node/store/src/partial_beacon_state.rs
+++ b/beacon_node/store/src/partial_beacon_state.rs
@@ -79,7 +79,7 @@ where
 
     // Inactivity
     #[superstruct(only(Altair, Merge))]
-    pub inactivity_scores: VariableList<u64, T::ValidatorRegistryLimit>,
+    pub inactivity_scores: VList<u64, T::ValidatorRegistryLimit>,
 
     // Light-client sync committees
     #[superstruct(only(Altair, Merge))]
@@ -266,17 +266,9 @@ impl<T: EthSpec> PartialBeaconState<T> {
             let current_epoch = self.slot().epoch(T::slots_per_epoch());
             let len = randao_mixes.len();
 
-            #[cfg(feature = "milhouse")]
-            {
-                use milhouse::interface::MutList;
-                randao_mixes
-                    .replace(current_epoch.as_usize() % len, *self.latest_randao_value())?;
-            }
-
-            #[cfg(not(feature = "milhouse"))]
-            {
-                randao_mixes[current_epoch.as_usize() % len] = *self.latest_randao_value();
-            }
+            *randao_mixes
+                .get_mut(current_epoch.as_usize() % len)
+                .unwrap() = *self.latest_randao_value();
 
             *self.randao_mixes_mut() = Some(randao_mixes)
         }
diff --git a/consensus/state_processing/src/common/initiate_validator_exit.rs b/consensus/state_processing/src/common/initiate_validator_exit.rs
index fbf4d037ab..a2bb47ebe0 100644
--- a/consensus/state_processing/src/common/initiate_validator_exit.rs
+++ b/consensus/state_processing/src/common/initiate_validator_exit.rs
@@ -32,13 +32,11 @@ pub fn initiate_validator_exit<T: EthSpec>(
         .exit_cache_mut()
         .record_validator_exit(exit_queue_epoch)?;
 
-    let mut validators = state.validators_mut();
-    let validator = validators.get_validator_mut(index)?;
-
+    // FIXME(sproul): could avoid this second lookup with some clever borrowing
+    let mut validator = state.get_validator_mut(index)?;
     validator.exit_epoch = exit_queue_epoch;
     validator.withdrawable_epoch =
         exit_queue_epoch.safe_add(spec.min_validator_withdrawability_delay)?;
-    drop(validators);
 
     Ok(())
 }
diff --git a/consensus/state_processing/src/common/mod.rs b/consensus/state_processing/src/common/mod.rs
index a1e4f673b8..334a293ed5 100644
--- a/consensus/state_processing/src/common/mod.rs
+++ b/consensus/state_processing/src/common/mod.rs
@@ -16,7 +16,7 @@ pub use initiate_validator_exit::initiate_validator_exit;
 pub use slash_validator::slash_validator;
 
 use safe_arith::SafeArith;
-use types::{BeaconState, BeaconStateError, EthSpec, GetBalanceMut};
+use types::{BeaconState, BeaconStateError, EthSpec};
 
 /// Increase the balance of a validator, erroring upon overflow, as per the spec.
 pub fn increase_balance<E: EthSpec>(
@@ -24,10 +24,7 @@ pub fn increase_balance<E: EthSpec>(
     index: usize,
     delta: u64,
 ) -> Result<(), BeaconStateError> {
-    state
-        .balances_mut()
-        .get_balance_mut(index)?
-        .safe_add_assign(delta)?;
+    state.get_balance_mut(index)?.safe_add_assign(delta)?;
     Ok(())
 }
 
@@ -37,8 +34,7 @@ pub fn decrease_balance<E: EthSpec>(
     index: usize,
     delta: u64,
 ) -> Result<(), BeaconStateError> {
-    let mut balances = state.balances_mut();
-    let balance = balances.get_balance_mut(index)?;
+    let balance = state.get_balance_mut(index)?;
     *balance = balance.saturating_sub(delta);
     Ok(())
 }
diff --git a/consensus/state_processing/src/common/slash_validator.rs b/consensus/state_processing/src/common/slash_validator.rs
index 5a03ea0322..e9d94a1062 100644
--- a/consensus/state_processing/src/common/slash_validator.rs
+++ b/consensus/state_processing/src/common/slash_validator.rs
@@ -17,16 +17,13 @@ pub fn slash_validator<T: EthSpec>(
 
     initiate_validator_exit(state, slashed_index, spec)?;
 
-    let mut validators = state.validators_mut();
-    let validator = validators.get_validator_mut(slashed_index)?;
+    let validator = state.get_validator_mut(slashed_index)?;
     validator.slashed = true;
     validator.withdrawable_epoch = cmp::max(
         validator.withdrawable_epoch,
         epoch.safe_add(T::EpochsPerSlashingsVector::to_u64())?,
     );
     let validator_effective_balance = validator.effective_balance;
-    drop(validators);
-
     state.set_slashings(
         epoch,
         state
diff --git a/consensus/state_processing/src/genesis.rs b/consensus/state_processing/src/genesis.rs
index 44c008276e..6680c615db 100644
--- a/consensus/state_processing/src/genesis.rs
+++ b/consensus/state_processing/src/genesis.rs
@@ -96,9 +96,10 @@ pub fn process_activations<T: EthSpec>(
     state: &mut BeaconState<T>,
     spec: &ChainSpec,
 ) -> Result<(), Error> {
-    let (mut validators, balances) = state.validators_and_balances_mut();
-    for index in 0..validators.len() {
-        let validator = validators.get_validator_mut(index)?;
+    let (validators, balances) = state.validators_and_balances_mut();
+    let mut validators_iter = validators.iter_cow();
+    while let Some((index, validator)) = validators_iter.next_cow() {
+        let validator = validator.to_mut();
         let balance = balances
             .get(index)
             .copied()
diff --git a/consensus/state_processing/src/per_epoch_processing/altair/participation_cache.rs b/consensus/state_processing/src/per_epoch_processing/altair/participation_cache.rs
index 503dadfc70..47105cff15 100644
--- a/consensus/state_processing/src/per_epoch_processing/altair/participation_cache.rs
+++ b/consensus/state_processing/src/per_epoch_processing/altair/participation_cache.rs
@@ -19,6 +19,7 @@ use types::{
         TIMELY_TARGET_FLAG_INDEX,
     },
     BeaconState, BeaconStateError, ChainSpec, Epoch, EthSpec, ParticipationFlags, RelativeEpoch,
+    Validator,
 };
 
 #[derive(Debug, PartialEq)]
@@ -120,12 +121,10 @@ impl SingleEpochParticipationCache {
     fn process_active_validator<T: EthSpec>(
         &mut self,
         val_index: usize,
+        validator: &Validator,
         state: &BeaconState<T>,
         relative_epoch: RelativeEpoch,
     ) -> Result<(), BeaconStateError> {
-        let val_balance = state.get_effective_balance(val_index)?;
-        let validator = state.get_validator(val_index)?;
-
         // Sanity check to ensure the validator is active.
         let epoch = relative_epoch.into_epoch(state.current_epoch());
         if !validator.is_active_at(epoch) {
@@ -141,7 +140,8 @@ impl SingleEpochParticipationCache {
         .ok_or(BeaconStateError::ParticipationOutOfBounds(val_index))?;
 
         // All active validators increase the total active balance.
-        self.total_active_balance.safe_add_assign(val_balance)?;
+        self.total_active_balance
+            .safe_add_assign(validator.effective_balance)?;
 
         // Only unslashed validators may proceed.
         if validator.slashed {
@@ -156,7 +156,7 @@ impl SingleEpochParticipationCache {
         // are set for `val_index`.
         for (flag, balance) in self.total_flag_balances.iter_mut().enumerate() {
             if epoch_participation.has_flag(flag)? {
-                balance.safe_add_assign(val_balance)?;
+                balance.safe_add_assign(validator.effective_balance)?;
             }
         }
 
@@ -223,6 +223,7 @@ impl ParticipationCache {
             if val.is_active_at(current_epoch) {
                 current_epoch_participation.process_active_validator(
                     val_index,
+                    val,
                     state,
                     RelativeEpoch::Current,
                 )?;
@@ -231,6 +232,7 @@ impl ParticipationCache {
             if val.is_active_at(previous_epoch) {
                 previous_epoch_participation.process_active_validator(
                     val_index,
+                    val,
                     state,
                     RelativeEpoch::Previous,
                 )?;
diff --git a/consensus/state_processing/src/per_epoch_processing/altair/rewards_and_penalties.rs b/consensus/state_processing/src/per_epoch_processing/altair/rewards_and_penalties.rs
index b1c17851d1..defc1d572c 100644
--- a/consensus/state_processing/src/per_epoch_processing/altair/rewards_and_penalties.rs
+++ b/consensus/state_processing/src/per_epoch_processing/altair/rewards_and_penalties.rs
@@ -69,6 +69,7 @@ pub fn get_flag_index_deltas<T: EthSpec>(
     let active_increments = total_active_balance.safe_div(spec.effective_balance_increment)?;
 
     for &index in participation_cache.eligible_validator_indices() {
+        // FIXME(sproul): compute base reward in participation cache
         let base_reward = get_base_reward(state, index, total_active_balance, spec)?;
         let mut delta = Delta::default();
 
diff --git a/consensus/state_processing/src/per_epoch_processing/effective_balance_updates.rs b/consensus/state_processing/src/per_epoch_processing/effective_balance_updates.rs
index 5378bf47fb..c87871f1d8 100644
--- a/consensus/state_processing/src/per_epoch_processing/effective_balance_updates.rs
+++ b/consensus/state_processing/src/per_epoch_processing/effective_balance_updates.rs
@@ -2,7 +2,7 @@ use super::errors::EpochProcessingError;
 use safe_arith::SafeArith;
 use types::beacon_state::BeaconState;
 use types::chain_spec::ChainSpec;
-use types::{BeaconStateError, EthSpec, GetValidatorMut};
+use types::{BeaconStateError, EthSpec};
 
 pub fn process_effective_balance_updates<T: EthSpec>(
     state: &mut BeaconState<T>,
@@ -13,9 +13,9 @@ pub fn process_effective_balance_updates<T: EthSpec>(
         .safe_div(spec.hysteresis_quotient)?;
     let downward_threshold = hysteresis_increment.safe_mul(spec.hysteresis_downward_multiplier)?;
     let upward_threshold = hysteresis_increment.safe_mul(spec.hysteresis_upward_multiplier)?;
-    let (mut validators, balances) = state.validators_and_balances_mut();
-    for index in 0..validators.len() {
-        let validator = validators.get_validator_mut(index)?;
+    let (validators, balances) = state.validators_and_balances_mut();
+    let mut validators_iter = validators.iter_cow();
+    while let Some((index, validator)) = validators_iter.next_cow() {
         let balance = balances
             .get(index)
             .copied()
@@ -24,7 +24,7 @@ pub fn process_effective_balance_updates<T: EthSpec>(
         if balance.safe_add(downward_threshold)? < validator.effective_balance
             || validator.effective_balance.safe_add(upward_threshold)? < balance
         {
-            validator.effective_balance = std::cmp::min(
+            validator.to_mut().effective_balance = std::cmp::min(
                 balance.safe_sub(balance.safe_rem(spec.effective_balance_increment)?)?,
                 spec.max_effective_balance,
             );
diff --git a/consensus/state_processing/src/per_epoch_processing/registry_updates.rs b/consensus/state_processing/src/per_epoch_processing/registry_updates.rs
index 93daa7def2..4fd2d68586 100644
--- a/consensus/state_processing/src/per_epoch_processing/registry_updates.rs
+++ b/consensus/state_processing/src/per_epoch_processing/registry_updates.rs
@@ -1,7 +1,7 @@
 use crate::{common::initiate_validator_exit, per_epoch_processing::Error};
 use itertools::Itertools;
 use safe_arith::SafeArith;
-use types::{BeaconState, ChainSpec, EthSpec, GetValidatorMut, Validator};
+use types::{BeaconState, ChainSpec, EthSpec, Validator};
 
 /// Performs a validator registry update, if required.
 ///
@@ -30,13 +30,11 @@ pub fn process_registry_updates<T: EthSpec>(
         .collect();
 
     for index in indices_to_update {
-        let mut validators = state.validators_mut();
-        let validator = validators.get_validator_mut(index)?;
+        let validator = state.get_validator_mut(index)?;
         if validator.is_eligible_for_activation_queue(spec) {
             validator.activation_eligibility_epoch = current_epoch.safe_add(1)?;
         }
         if is_ejectable(validator) {
-            drop(validators);
             initiate_validator_exit(state, index, spec)?;
         }
     }
@@ -54,9 +52,8 @@ pub fn process_registry_updates<T: EthSpec>(
     // Dequeue validators for activation up to churn limit
     let churn_limit = state.get_churn_limit(spec)? as usize;
     let delayed_activation_epoch = state.compute_activation_exit_epoch(current_epoch, spec)?;
-    let mut validators = state.validators_mut();
     for index in activation_queue.into_iter().take(churn_limit) {
-        validators.get_validator_mut(index)?.activation_epoch = delayed_activation_epoch;
+        state.get_validator_mut(index)?.activation_epoch = delayed_activation_epoch;
     }
 
     Ok(())
diff --git a/consensus/state_processing/src/per_epoch_processing/slashings.rs b/consensus/state_processing/src/per_epoch_processing/slashings.rs
index 69807a5e5f..e1632280f4 100644
--- a/consensus/state_processing/src/per_epoch_processing/slashings.rs
+++ b/consensus/state_processing/src/per_epoch_processing/slashings.rs
@@ -1,6 +1,6 @@
 use crate::per_epoch_processing::Error;
 use safe_arith::{SafeArith, SafeArithIter};
-use types::{BeaconState, ChainSpec, EthSpec, GetBalanceMut, GetValidatorMut, Unsigned};
+use types::{BeaconState, BeaconStateError, ChainSpec, EthSpec, Unsigned};
 
 /// Process slashings.
 pub fn process_slashings<T: EthSpec>(
@@ -16,9 +16,9 @@ pub fn process_slashings<T: EthSpec>(
         total_balance,
     );
 
-    let (validators, mut balances) = state.validators_and_balances_mut();
-    for index in 0..validators.len() {
-        let validator = validators.get_validator(index)?;
+    let (validators, balances) = state.validators_and_balances_mut();
+    let mut validators_iter = validators.iter_cow();
+    while let Some((index, validator)) = validators_iter.next_cow() {
         if validator.slashed
             && epoch.safe_add(T::EpochsPerSlashingsVector::to_u64().safe_div(2)?)?
                 == validator.withdrawable_epoch
@@ -33,7 +33,9 @@ pub fn process_slashings<T: EthSpec>(
                 .safe_mul(increment)?;
 
             // Equivalent to `decrease_balance(state, index, penalty)`, but avoids borrowing `state`.
-            let balance = balances.get_balance_mut(index)?;
+            let balance = balances
+                .get_mut(index)
+                .ok_or(BeaconStateError::BalancesOutOfBounds(index))?;
             *balance = balance.saturating_sub(penalty);
         }
     }
diff --git a/consensus/state_processing/src/upgrade/altair.rs b/consensus/state_processing/src/upgrade/altair.rs
index b32afc5ee3..94c65e270a 100644
--- a/consensus/state_processing/src/upgrade/altair.rs
+++ b/consensus/state_processing/src/upgrade/altair.rs
@@ -51,7 +51,7 @@ pub fn upgrade_to_altair<E: EthSpec>(
 
     let default_epoch_participation =
         VariableList::new(vec![ParticipationFlags::default(); pre.validators.len()])?;
-    let inactivity_scores = VariableList::new(vec![0; pre.validators.len()])?;
+    let inactivity_scores = VList::new(vec![0; pre.validators.len()])?;
 
     let temp_sync_committee = Arc::new(SyncCommittee::temporary()?);
 
diff --git a/consensus/types/src/beacon_state.rs b/consensus/types/src/beacon_state.rs
index ab02257e82..69d0a5ca39 100644
--- a/consensus/types/src/beacon_state.rs
+++ b/consensus/types/src/beacon_state.rs
@@ -47,28 +47,6 @@ mod tests;
 #[cfg(not(feature = "milhouse"))]
 mod tree_hash_cache;
 
-#[cfg(feature = "milhouse")]
-mod type_aliases {
-    use super::*;
-
-    pub type ListMut<'a, T, N> = Interface<'a, T, List<T, N>>;
-    pub type VectMut<'a, T, N> = Interface<'a, T, FixedVector<T, N>>;
-    pub type ValidatorsMut<'a, N> = ListMut<'a, Validator, N>;
-    pub type BalancesMut<'a, N> = ListMut<'a, u64, N>;
-}
-
-#[cfg(not(feature = "milhouse"))]
-mod type_aliases {
-    use super::*;
-
-    pub type ListMut<'a, T, N> = &'a mut VList<T, N>;
-    pub type VectMut<'a, T, N> = &'a mut FixedVector<T, N>;
-    pub type ValidatorsMut<'a, N> = &'a mut VList<Validator, N>;
-    pub type BalancesMut<'a, N> = ListMut<'a, u64, N>;
-}
-
-pub use type_aliases::*;
-
 pub const CACHED_EPOCHS: usize = 3;
 const MAX_RANDOM_BYTE: u64 = (1 << 8) - 1;
 
@@ -248,10 +226,8 @@ where
 
     // History
     pub latest_block_header: BeaconBlockHeader,
-    #[superstruct(getter(rename = "block_roots_raw"))]
     #[test_random(default)]
     pub block_roots: FixedVector<Hash256, T::SlotsPerHistoricalRoot>,
-    #[superstruct(getter(rename = "state_roots_raw"))]
     #[test_random(default)]
     pub state_roots: FixedVector<Hash256, T::SlotsPerHistoricalRoot>,
     #[test_random(default)]
@@ -266,22 +242,18 @@ where
     pub eth1_deposit_index: u64,
 
     // Registry
-    #[superstruct(getter(rename = "validators_raw"))]
     #[test_random(default)]
     pub validators: VList<Validator, T::ValidatorRegistryLimit>,
     // FIXME(sproul): serde quoting
     // #[serde(with = "ssz_types::serde_utils::quoted_u64_var_list")]
-    #[superstruct(getter(rename = "balances_raw"))]
     #[test_random(default)]
     pub balances: VList<u64, T::ValidatorRegistryLimit>,
 
     // Randomness
-    #[superstruct(getter(rename = "randao_mixes_raw"))]
     #[test_random(default)]
     pub randao_mixes: FixedVector<Hash256, T::EpochsPerHistoricalVector>,
 
     // Slashings
-    #[superstruct(getter(rename = "slashings_raw"))]
     #[test_random(default)]
     // FIXME(sproul): serde quoting
     // #[serde(with = "ssz_types::serde_utils::quoted_u64_fixed_vec")]
@@ -312,9 +284,11 @@ where
     pub finalized_checkpoint: Checkpoint,
 
     // Inactivity
-    #[serde(with = "ssz_types::serde_utils::quoted_u64_var_list")]
+    // FIXME(sproul): quoting
+    // #[serde(with = "ssz_types::serde_utils::quoted_u64_var_list")]
     #[superstruct(only(Altair, Merge))]
-    pub inactivity_scores: VariableList<u64, T::ValidatorRegistryLimit>,
+    #[test_random(default)]
+    pub inactivity_scores: VList<u64, T::ValidatorRegistryLimit>,
 
     // Light-client sync committees
     #[superstruct(only(Altair, Merge))]
@@ -990,7 +964,7 @@ impl<T: EthSpec> BeaconState<T> {
 
     /// Fill `randao_mixes` with
     pub fn fill_randao_mixes_with(&mut self, index_root: Hash256) {
-        *self.randao_mixes_raw_mut() = FixedVector::from_elem(index_root);
+        *self.randao_mixes_mut() = FixedVector::from_elem(index_root);
     }
 
     /// Safely obtains the index for `randao_mixes`
@@ -1139,116 +1113,25 @@ impl<T: EthSpec> BeaconState<T> {
         Ok(())
     }
 
-    pub fn validators(&self) -> &VList<Validator, T::ValidatorRegistryLimit> {
-        self.validators_raw()
-    }
-
-    pub fn validators_mut(&mut self) -> ValidatorsMut<T::ValidatorRegistryLimit> {
-        #[cfg(not(feature = "milhouse"))]
-        {
-            self.validators_raw_mut()
-        }
-        #[cfg(feature = "milhouse")]
-        {
-            self.validators_raw_mut().as_mut()
-        }
-    }
-
-    pub fn balances(&self) -> &VList<u64, T::ValidatorRegistryLimit> {
-        self.balances_raw()
-    }
-
-    pub fn balances_mut(&mut self) -> BalancesMut<T::ValidatorRegistryLimit> {
-        #[cfg(not(feature = "milhouse"))]
-        {
-            self.balances_raw_mut()
-        }
-        #[cfg(feature = "milhouse")]
-        {
-            self.balances_raw_mut().as_mut()
-        }
-    }
-
     /// Convenience accessor for validators and balances simultaneously.
     pub fn validators_and_balances_mut(
         &mut self,
     ) -> (
-        ValidatorsMut<T::ValidatorRegistryLimit>,
-        BalancesMut<T::ValidatorRegistryLimit>,
+        &mut VList<Validator, T::ValidatorRegistryLimit>,
+        &mut VList<u64, T::ValidatorRegistryLimit>,
     ) {
-        #[cfg(not(feature = "milhouse"))]
         match self {
             BeaconState::Base(state) => (&mut state.validators, &mut state.balances),
             BeaconState::Altair(state) => (&mut state.validators, &mut state.balances),
             BeaconState::Merge(state) => (&mut state.validators, &mut state.balances),
         }
-
-        #[cfg(feature = "milhouse")]
-        match self {
-            BeaconState::Base(state) => (state.validators.as_mut(), state.balances.as_mut()),
-            BeaconState::Altair(state) => (state.validators.as_mut(), state.balances.as_mut()),
-            BeaconState::Merge(state) => (state.validators.as_mut(), state.balances.as_mut()),
-        }
     }
 
-    pub fn block_roots(&self) -> &FixedVector<Hash256, T::SlotsPerHistoricalRoot> {
-        self.block_roots_raw()
-    }
-
-    pub fn block_roots_mut(&mut self) -> VectMut<Hash256, T::SlotsPerHistoricalRoot> {
-        #[cfg(not(feature = "milhouse"))]
-        {
-            self.block_roots_raw_mut()
-        }
-        #[cfg(feature = "milhouse")]
-        {
-            self.block_roots_raw_mut().as_mut()
-        }
-    }
-
-    pub fn state_roots(&self) -> &FixedVector<Hash256, T::SlotsPerHistoricalRoot> {
-        self.state_roots_raw()
-    }
-
-    pub fn state_roots_mut(&mut self) -> VectMut<Hash256, T::SlotsPerHistoricalRoot> {
-        #[cfg(not(feature = "milhouse"))]
-        {
-            self.state_roots_raw_mut()
-        }
-        #[cfg(feature = "milhouse")]
-        {
-            self.state_roots_raw_mut().as_mut()
-        }
-    }
-
-    pub fn randao_mixes(&self) -> &FixedVector<Hash256, T::EpochsPerHistoricalVector> {
-        self.randao_mixes_raw()
-    }
-
-    pub fn randao_mixes_mut(&mut self) -> VectMut<Hash256, T::EpochsPerHistoricalVector> {
-        #[cfg(not(feature = "milhouse"))]
-        {
-            self.randao_mixes_raw_mut()
-        }
-        #[cfg(feature = "milhouse")]
-        {
-            self.randao_mixes_raw_mut().as_mut()
-        }
-    }
-
-    pub fn slashings(&self) -> &FixedVector<u64, T::EpochsPerSlashingsVector> {
-        self.slashings_raw()
-    }
-
-    pub fn slashings_mut(&mut self) -> VectMut<u64, T::EpochsPerSlashingsVector> {
-        #[cfg(not(feature = "milhouse"))]
-        {
-            self.slashings_raw_mut()
-        }
-        #[cfg(feature = "milhouse")]
-        {
-            self.slashings_raw_mut().as_mut()
-        }
+    /// Get a mutable reference to the balance of a single validator.
+    pub fn get_balance_mut(&mut self, validator_index: usize) -> Result<&mut u64, Error> {
+        self.balances_mut()
+            .get_mut(validator_index)
+            .ok_or(Error::BalancesOutOfBounds(validator_index))
     }
 
     /// Generate a seed for the given `epoch`.
@@ -1293,14 +1176,22 @@ impl<T: EthSpec> BeaconState<T> {
             .ok_or(Error::UnknownValidator(validator_index))
     }
 
-    /* FIXME(sproul): lens?
     /// Safe mutator for the `validators` list.
     pub fn get_validator_mut(&mut self, validator_index: usize) -> Result<&mut Validator, Error> {
         self.validators_mut()
             .get_mut(validator_index)
             .ok_or(Error::UnknownValidator(validator_index))
     }
-    */
+
+    /// Safe copy-on-write accessor for the `validators` list.
+    pub fn get_validator_cow(
+        &mut self,
+        validator_index: usize,
+    ) -> Result<milhouse::Cow<Validator>, Error> {
+        self.validators_mut()
+            .get_cow(validator_index)
+            .ok_or(Error::UnknownValidator(validator_index))
+    }
 
     /// Return the effective balance for a validator with the given `validator_index`.
     pub fn get_effective_balance(&self, validator_index: usize) -> Result<u64, Error> {
@@ -1387,6 +1278,28 @@ impl<T: EthSpec> BeaconState<T> {
         ))
     }
 
+    pub fn compute_total_active_balance(
+        &self,
+        epoch: Epoch,
+        spec: &ChainSpec,
+    ) -> Result<u64, Error> {
+        if epoch != self.current_epoch() && epoch != self.next_epoch()? {
+            return Err(Error::EpochOutOfBounds);
+        }
+
+        let mut total_active_balance = 0;
+
+        for validator in self.validators() {
+            if validator.is_active_at(epoch) {
+                total_active_balance.safe_add_assign(validator.effective_balance)?;
+            }
+        }
+        Ok(std::cmp::max(
+            total_active_balance,
+            spec.effective_balance_increment,
+        ))
+    }
+
     /// Implementation of `get_total_active_balance`, matching the spec.
     ///
     /// Requires the total active balance cache to be initialised, which is initialised whenever
@@ -1410,16 +1323,9 @@ impl<T: EthSpec> BeaconState<T> {
     }
 
     /// Build the total active balance cache.
-    ///
-    /// This function requires the current committee cache to be already built. It is called
-    /// automatically when `build_committee_cache` is called for the current epoch.
     fn build_total_active_balance_cache(&mut self, spec: &ChainSpec) -> Result<(), Error> {
-        // Order is irrelevant, so use the cached indices.
         let current_epoch = self.current_epoch();
-        let total_active_balance = self.get_total_balance(
-            self.get_cached_active_validator_indices(RelativeEpoch::Current)?,
-            spec,
-        )?;
+        let total_active_balance = self.compute_total_active_balance(current_epoch, spec)?;
         *self.total_active_balance_mut() = Some((current_epoch, total_active_balance));
         Ok(())
     }
@@ -1578,10 +1484,9 @@ impl<T: EthSpec> BeaconState<T> {
         // not yet been advanced.
         let new_current_epoch = self.next_epoch()?;
         if curr_cache.is_initialized_at(new_current_epoch) {
-            *self.total_active_balance_mut() = Some((
-                new_current_epoch,
-                self.get_total_balance(curr_cache.active_validator_indices(), spec)?,
-            ));
+            let total_active_balance =
+                self.compute_total_active_balance(new_current_epoch, spec)?;
+            *self.total_active_balance_mut() = Some((new_current_epoch, total_active_balance));
         }
         // If the cache is not initialized, then the previous cached value for the total balance is
         // wrong, so delete it.
@@ -1671,6 +1576,38 @@ impl<T: EthSpec> BeaconState<T> {
         *self.pubkey_cache_mut() = PubkeyCache::default()
     }
 
+    /// Check if the `BeaconState` has any pending mutations.
+    pub fn has_pending_mutations(&self) -> bool {
+        // FIXME(sproul): check this more thoroughly
+        self.block_roots().has_pending_updates()
+            || self.state_roots().has_pending_updates()
+            || self.historical_roots().has_pending_updates()
+            || self.eth1_data_votes().has_pending_updates()
+            || self.validators().has_pending_updates()
+            || self.balances().has_pending_updates()
+            || self.randao_mixes().has_pending_updates()
+            || self.slashings().has_pending_updates()
+            || self
+                .inactivity_scores()
+                .map_or(false, VList::has_pending_updates)
+    }
+
+    pub fn apply_pending_mutations(&mut self) -> Result<(), Error> {
+        self.block_roots_mut().apply_updates()?;
+        self.state_roots_mut().apply_updates()?;
+        self.historical_roots_mut().apply_updates()?;
+        self.eth1_data_votes_mut().apply_updates()?;
+        self.validators_mut().apply_updates()?;
+        self.balances_mut().apply_updates()?;
+        self.randao_mixes_mut().apply_updates()?;
+        self.slashings_mut().apply_updates()?;
+
+        if let Ok(inactivity_scores) = self.inactivity_scores_mut() {
+            inactivity_scores.apply_updates()?;
+        }
+        Ok(())
+    }
+
     /// Initialize but don't fill the tree hash cache, if it isn't already initialized.
     pub fn initialize_tree_hash_cache(&mut self) {
         #[cfg(not(feature = "milhouse"))]
@@ -1700,7 +1637,10 @@ impl<T: EthSpec> BeaconState<T> {
             }
         }
         #[cfg(feature = "milhouse")]
-        Ok(self.tree_hash_root())
+        {
+            self.apply_pending_mutations()?;
+            Ok(self.tree_hash_root())
+        }
     }
 
     /// Compute the tree hash root of the validators using the tree hash cache.
@@ -1724,7 +1664,10 @@ impl<T: EthSpec> BeaconState<T> {
             }
         }
         #[cfg(feature = "milhouse")]
-        Ok(self.validators().tree_hash_root())
+        {
+            self.validators_mut().apply_updates()?;
+            Ok(self.validators().tree_hash_root())
+        }
     }
 
     /// Completely drops the tree hash cache, replacing it with a new, empty cache.
diff --git a/consensus/types/src/lib.rs b/consensus/types/src/lib.rs
index 695ecbc101..ef0fcf804c 100644
--- a/consensus/types/src/lib.rs
+++ b/consensus/types/src/lib.rs
@@ -66,7 +66,6 @@ pub mod voluntary_exit;
 pub mod slot_epoch_macros;
 pub mod config_and_preset;
 pub mod fork_context;
-pub mod mixin;
 pub mod participation_flags;
 pub mod participation_list;
 pub mod preset;
@@ -123,7 +122,6 @@ pub use crate::free_attestation::FreeAttestation;
 pub use crate::graffiti::{Graffiti, GRAFFITI_BYTES_LEN};
 pub use crate::historical_batch::HistoricalBatch;
 pub use crate::indexed_attestation::IndexedAttestation;
-pub use crate::mixin::{GetBalanceMut, GetValidatorMut};
 pub use crate::participation_flags::ParticipationFlags;
 pub use crate::participation_list::ParticipationList;
 pub use crate::pending_attestation::PendingAttestation;
diff --git a/consensus/types/src/mixin.rs b/consensus/types/src/mixin.rs
deleted file mode 100644
index 41f2203740..0000000000
--- a/consensus/types/src/mixin.rs
+++ /dev/null
@@ -1,36 +0,0 @@
-use crate::beacon_state::{BalancesMut, Error, ValidatorsMut};
-use crate::{Unsigned, Validator};
-
-pub trait GetValidatorMut {
-    fn get_validator(&self, index: usize) -> Result<&Validator, Error>;
-
-    fn get_validator_mut(&mut self, index: usize) -> Result<&mut Validator, Error>;
-}
-
-impl<'a, N: Unsigned> GetValidatorMut for ValidatorsMut<'a, N> {
-    fn get_validator(&self, index: usize) -> Result<&Validator, Error> {
-        self.get(index).ok_or(Error::UnknownValidator(index))
-    }
-
-    fn get_validator_mut(&mut self, index: usize) -> Result<&mut Validator, Error> {
-        self.get_mut(index).ok_or(Error::UnknownValidator(index))
-    }
-}
-
-pub trait GetBalanceMut {
-    fn get_balance(&self, index: usize) -> Result<u64, Error>;
-
-    fn get_balance_mut(&mut self, index: usize) -> Result<&mut u64, Error>;
-}
-
-impl<'a, N: Unsigned> GetBalanceMut for BalancesMut<'a, N> {
-    fn get_balance(&self, index: usize) -> Result<u64, Error> {
-        self.get(index)
-            .copied()
-            .ok_or(Error::BalancesOutOfBounds(index))
-    }
-
-    fn get_balance_mut(&mut self, index: usize) -> Result<&mut u64, Error> {
-        self.get_mut(index).ok_or(Error::BalancesOutOfBounds(index))
-    }
-}
diff --git a/lcli/src/replace_state_pubkeys.rs b/lcli/src/replace_state_pubkeys.rs
index bba391aca8..97ef94e32a 100644
--- a/lcli/src/replace_state_pubkeys.rs
+++ b/lcli/src/replace_state_pubkeys.rs
@@ -42,7 +42,7 @@ pub fn run<T: EthSpec>(testnet_dir: PathBuf, matches: &ArgMatches) -> Result<(),
 
     let mut deposit_tree = DepositDataTree::create(&[], 0, DEPOSIT_TREE_DEPTH);
     let mut deposit_root = Hash256::zero();
-    let mut validators = state.validators_mut();
+    let validators = state.validators_mut();
     for index in 0..validators.len() {
         let (secret, _) =
             recover_validator_secret_from_mnemonic(seed.as_bytes(), index as u32, KeyType::Voting)