diff --git a/beacon_chain/spec/beaconstate.nim b/beacon_chain/spec/beaconstate.nim
index 10dbb8a56..efda9fa7d 100644
--- a/beacon_chain/spec/beaconstate.nim
+++ b/beacon_chain/spec/beaconstate.nim
@@ -118,16 +118,16 @@ func compute_activation_exit_epoch(epoch: Epoch): Epoch =
   epoch + 1 + MAX_SEED_LOOKAHEAD
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#get_validator_churn_limit
-func get_validator_churn_limit(state: BeaconState): uint64 =
+func get_validator_churn_limit(state: BeaconState, cache: var StateCache):
+    uint64 =
   # Return the validator churn limit for the current epoch.
-  let active_validator_indices =
-    get_active_validator_indices(state, get_current_epoch(state))
   max(MIN_PER_EPOCH_CHURN_LIMIT,
-    len(active_validator_indices) div CHURN_LIMIT_QUOTIENT).uint64
+    len(cache.shuffled_active_validator_indices) div
+      CHURN_LIMIT_QUOTIENT).uint64
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#initiate_validator_exit
 func initiate_validator_exit*(state: var BeaconState,
-                              index: ValidatorIndex) =
+                              index: ValidatorIndex, cache: var StateCache) =
   # Initiate the exit of the validator with index ``index``.
 
   # Return if validator already initiated exit
@@ -146,7 +146,7 @@ func initiate_validator_exit*(state: var BeaconState,
     a + (if b.exit_epoch == exit_queue_epoch: 1'u64 else: 0'u64),
     0'u64)
 
-  if exit_queue_churn >= get_validator_churn_limit(state):
+  if exit_queue_churn >= get_validator_churn_limit(state, cache):
     exit_queue_epoch += 1
 
   # Set validator exit epoch and withdrawable epoch
@@ -156,10 +156,10 @@ func initiate_validator_exit*(state: var BeaconState,
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#slash_validator
 proc slash_validator*(state: var BeaconState, slashed_index: ValidatorIndex,
-    stateCache: var StateCache) =
+    cache: var StateCache) =
   # Slash the validator with index ``index``.
   let epoch = get_current_epoch(state)
-  initiate_validator_exit(state, slashed_index)
+  initiate_validator_exit(state, slashed_index, cache)
   let validator = addr state.validators[slashed_index]
 
   debug "slash_validator: ejecting validator via slashing (validator_leaving)",
@@ -181,7 +181,7 @@ proc slash_validator*(state: var BeaconState, slashed_index: ValidatorIndex,
 
   # The rest doesn't make sense without there being any proposer index, so skip
   # Apply proposer and whistleblower rewards
-  let proposer_index = get_beacon_proposer_index(state, stateCache)
+  let proposer_index = get_beacon_proposer_index(state, cache)
   if proposer_index.isNone:
     debug "No beacon proposer index and probably no active validators"
     return
@@ -282,6 +282,7 @@ proc initialize_hashed_beacon_state_from_eth1*(
 func is_valid_genesis_state*(state: BeaconState): bool =
   if state.genesis_time < MIN_GENESIS_TIME:
     return false
+  # This is an okay get_active_validator_indices(...) for the time being.
   if len(get_active_validator_indices(state, GENESIS_EPOCH)) < MIN_GENESIS_ACTIVE_VALIDATOR_COUNT:
     return false
   return true
@@ -341,7 +342,8 @@ func is_eligible_for_activation(state: BeaconState, validator: Validator):
     validator.activation_epoch == FAR_FUTURE_EPOCH
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#registry-updates
-proc process_registry_updates*(state: var BeaconState) {.nbench.}=
+proc process_registry_updates*(state: var BeaconState,
+    cache: var StateCache) {.nbench.}=
   ## Process activation eligibility and ejections
   ## Try to avoid caching here, since this could easily become undefined
 
@@ -354,6 +356,12 @@ proc process_registry_updates*(state: var BeaconState) {.nbench.}=
     active_validator_indices=get_active_validator_indices(state, epoch),
     epoch=epoch
 
+  # is_active_validator(...) is activation_epoch <= epoch < exit_epoch,
+  # and changes here to either activation_epoch or exit_epoch only take
+  # effect with a compute_activation_exit_epoch(...) delay of, based on
+  # the current epoch, 1 + MAX_SEED_LOOKAHEAD epochs ahead. Thus caches
+  # remain valid for this epoch through though this function along with
+  # the rest of the epoch transition.
   for index, validator in state.validators:
     if is_eligible_for_activation_queue(validator):
       state.validators[index].activation_eligibility_epoch =
@@ -369,7 +377,7 @@ proc process_registry_updates*(state: var BeaconState) {.nbench.}=
         validator_withdrawable_epoch = validator.withdrawable_epoch,
         validator_exit_epoch = validator.exit_epoch,
         validator_effective_balance = validator.effective_balance
-      initiate_validator_exit(state, index.ValidatorIndex)
+      initiate_validator_exit(state, index.ValidatorIndex, cache)
 
   ## Queue validators eligible for activation and not dequeued for activation
   var activation_queue : seq[tuple[a: Epoch, b: int]] = @[]
@@ -382,7 +390,7 @@ proc process_registry_updates*(state: var BeaconState) {.nbench.}=
 
   ## Dequeued validators for activation up to churn limit (without resetting
   ## activation epoch)
-  let churn_limit = get_validator_churn_limit(state)
+  let churn_limit = get_validator_churn_limit(state, cache)
   for i, epoch_and_index in activation_queue:
     if i.uint64 >= churn_limit:
       break
diff --git a/beacon_chain/spec/datatypes.nim b/beacon_chain/spec/datatypes.nim
index 6e5ad3a8d..413ece23b 100644
--- a/beacon_chain/spec/datatypes.nim
+++ b/beacon_chain/spec/datatypes.nim
@@ -490,6 +490,7 @@ proc `[]=`*[T](a: var seq[T], b: ValidatorIndex, c: T) =
 
 # `ValidatorIndex` Nim integration
 proc `==`*(x, y: ValidatorIndex) : bool {.borrow.}
+proc `<`*(x, y: ValidatorIndex) : bool {.borrow.}
 proc hash*(x: ValidatorIndex): Hash {.borrow.}
 proc `$`*(x: ValidatorIndex): auto = $(x.int64)
 
diff --git a/beacon_chain/spec/state_transition_block.nim b/beacon_chain/spec/state_transition_block.nim
index dec2a1caf..99b6509e0 100644
--- a/beacon_chain/spec/state_transition_block.nim
+++ b/beacon_chain/spec/state_transition_block.nim
@@ -303,7 +303,9 @@ proc process_voluntary_exit*(
     validator_withdrawable_epoch = validator.withdrawable_epoch,
     validator_exit_epoch = validator.exit_epoch,
     validator_effective_balance = validator.effective_balance
-  initiate_validator_exit(state, voluntary_exit.validator_index.ValidatorIndex)
+  var cache = get_empty_per_epoch_cache()
+  initiate_validator_exit(
+    state, voluntary_exit.validator_index.ValidatorIndex, cache)
 
   true
 
diff --git a/beacon_chain/spec/state_transition_epoch.nim b/beacon_chain/spec/state_transition_epoch.nim
index 50fcf0148..4e6c39838 100644
--- a/beacon_chain/spec/state_transition_epoch.nim
+++ b/beacon_chain/spec/state_transition_epoch.nim
@@ -67,14 +67,23 @@ declareGauge beacon_current_epoch, "Current epoch"
 # --------------------------------------------------------
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#get_total_active_balance
-func get_total_active_balance*(state: BeaconState): Gwei =
+func get_total_active_balance*(state: BeaconState, cache: var StateCache): Gwei =
   # Return the combined effective balance of the active validators.
   # Note: ``get_total_balance`` returns ``EFFECTIVE_BALANCE_INCREMENT`` Gwei
   # minimum to avoid divisions by zero.
-  # TODO it calls get_total_balance with set(g_a_v_i(...))
-  get_total_balance(
-    state,
-    get_active_validator_indices(state, get_current_epoch(state)))
+
+  # TODO refactor get_epoch_per_epoch_cache() not to be, well, empty, so can
+  # avoid this ever refilling, and raiseAssert, and get rid of var
+  let
+    epoch = state.slot.compute_epoch_at_slot
+  try:
+    if epoch notin cache.shuffled_active_validator_indices:
+      cache.shuffled_active_validator_indices[epoch] =
+        get_shuffled_active_validator_indices(state, epoch)
+
+    get_total_balance(state, cache.shuffled_active_validator_indices[epoch])
+  except KeyError:
+    raiseAssert("get_total_active_balance(): cache always filled before usage")
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#helper-functions-1
 func get_matching_source_attestations(state: BeaconState,
@@ -155,6 +164,7 @@ proc process_justification_and_finalization*(state: var BeaconState,
   ## and
   ## https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#final-updates
   ## after which the state.previous_epoch_attestations is replaced.
+  let total_active_balance = get_total_active_balance(state, stateCache)
   trace "Non-attesting indices in previous epoch",
     missing_all_validators=
       difference(active_validator_indices,
@@ -167,10 +177,10 @@ proc process_justification_and_finalization*(state: var BeaconState,
     prev_attestations_len=len(state.previous_epoch_attestations),
     cur_attestations_len=len(state.current_epoch_attestations),
     num_active_validators=len(active_validator_indices),
-    required_balance = get_total_active_balance(state) * 2,
+    required_balance = total_active_balance * 2,
     attesting_balance_prev = get_attesting_balance(state, matching_target_attestations_previous, stateCache)
   if get_attesting_balance(state, matching_target_attestations_previous,
-      stateCache) * 3 >= get_total_active_balance(state) * 2:
+      stateCache) * 3 >= total_active_balance * 2:
     state.current_justified_checkpoint =
       Checkpoint(epoch: previous_epoch,
                  root: get_block_root(state, previous_epoch))
@@ -184,7 +194,7 @@ proc process_justification_and_finalization*(state: var BeaconState,
   let matching_target_attestations_current =
     get_matching_target_attestations(state, current_epoch)  # Current epoch
   if get_attesting_balance(state, matching_target_attestations_current,
-      stateCache) * 3 >= get_total_active_balance(state) * 2:
+      stateCache) * 3 >= total_active_balance * 2:
     state.current_justified_checkpoint =
       Checkpoint(epoch: current_epoch,
                  root: get_block_root(state, current_epoch))
@@ -256,7 +266,7 @@ func get_attestation_deltas(state: BeaconState, stateCache: var StateCache):
     tuple[a: seq[Gwei], b: seq[Gwei]] {.nbench.}=
   let
     previous_epoch = get_previous_epoch(state)
-    total_balance = get_total_active_balance(state)
+    total_balance = get_total_active_balance(state, stateCache)
   var
     rewards = repeat(0'u64, len(state.validators))
     penalties = repeat(0'u64, len(state.validators))
@@ -360,10 +370,10 @@ func process_rewards_and_penalties(
     decrease_balance(state, i.ValidatorIndex, penalties[i])
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.9.4/specs/core/0_beacon-chain.md#slashings
-func process_slashings*(state: var BeaconState) {.nbench.}=
+func process_slashings*(state: var BeaconState, cache: var StateCache) {.nbench.}=
   let
     epoch = get_current_epoch(state)
-    total_balance = get_total_active_balance(state)
+    total_balance = get_total_active_balance(state, cache)
 
   for index, validator in state.validators:
     if validator.slashed and epoch + EPOCHS_PER_SLASHINGS_VECTOR div 2 ==
@@ -443,16 +453,10 @@ proc process_epoch*(state: var BeaconState, updateFlags: UpdateFlags,
   process_rewards_and_penalties(state, per_epoch_cache)
 
   # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#registry-updates
-  # Don't rely on caching here.
-  process_registry_updates(state)
-
-  ## Caching here for get_beacon_committee(...) can break otherwise, since
-  ## get_active_validator_indices(...) usually changes.
-  per_epoch_cache.shuffled_active_validator_indices[currentEpoch] =
-    get_shuffled_active_validator_indices(state, currentEpoch)
+  process_registry_updates(state, per_epoch_cache)
 
   # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#slashings
-  process_slashings(state)
+  process_slashings(state, per_epoch_cache)
 
   # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#final-updates
   process_final_updates(state)
diff --git a/beacon_chain/spec/validator.nim b/beacon_chain/spec/validator.nim
index 6be71c582..ce6f1ad98 100644
--- a/beacon_chain/spec/validator.nim
+++ b/beacon_chain/spec/validator.nim
@@ -9,7 +9,7 @@
 {.push raises: [Defect].}
 
 import
-  options, sequtils, math, tables,
+  algorithm, options, sequtils, math, tables,
   ./datatypes, ./digest, ./helpers
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#compute_shuffled_index
@@ -138,7 +138,7 @@ func get_beacon_committee*(
   # missing cases here.
   if epoch notin cache.shuffled_active_validator_indices:
     cache.shuffled_active_validator_indices[epoch] =
-      get_shuffledactive_validator_indices(state, epoch)
+      get_shuffled_active_validator_indices(state, epoch)
 
   # Constant throughout an epoch
   if epoch notin cache.committee_count_cache:
@@ -164,7 +164,7 @@ func get_empty_per_epoch_cache*(): StateCache =
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#compute_proposer_index
 func compute_proposer_index(state: BeaconState, indices: seq[ValidatorIndex],
-    seed: Eth2Digest, stateCache: var StateCache): Option[ValidatorIndex] =
+    seed: Eth2Digest): Option[ValidatorIndex] =
   # Return from ``indices`` a random index sampled by effective balance.
   const MAX_RANDOM_BYTE = 255
 
@@ -194,7 +194,7 @@ func compute_proposer_index(state: BeaconState, indices: seq[ValidatorIndex],
     i += 1
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#get_beacon_proposer_index
-func get_beacon_proposer_index*(state: BeaconState, stateCache: var StateCache, slot: Slot):
+func get_beacon_proposer_index*(state: BeaconState, cache: var StateCache, slot: Slot):
     Option[ValidatorIndex] =
   # Return the beacon proposer index at the current slot.
   let epoch = get_current_epoch(state)
@@ -205,11 +205,19 @@ func get_beacon_proposer_index*(state: BeaconState, stateCache: var StateCache,
 
   # TODO fixme; should only be run once per slot and cached
   # There's exactly one beacon proposer per slot.
-  let
-    seed = eth2hash(buffer)
-    indices = get_active_validator_indices(state, epoch)
+  if epoch notin cache.shuffled_active_validator_indices:
+    cache.shuffled_active_validator_indices[epoch] =
+      get_shuffled_active_validator_indices(state, epoch)
 
-  compute_proposer_index(state, indices, seed, stateCache)
+  try:
+    let
+      seed = eth2hash(buffer)
+      indices =
+        sorted(cache.shuffled_active_validator_indices[epoch], system.cmp)
+
+    compute_proposer_index(state, indices, seed)
+  except KeyError:
+    raiseAssert("Cached entries are added before use")
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.3/specs/phase0/beacon-chain.md#get_beacon_proposer_index
 func get_beacon_proposer_index*(state: BeaconState, stateCache: var StateCache):
diff --git a/nbench/scenarios.nim b/nbench/scenarios.nim
index 14d55f1e9..9d83cbb87 100644
--- a/nbench/scenarios.nim
+++ b/nbench/scenarios.nim
@@ -1,5 +1,5 @@
 # beacon_chain
-# Copyright (c) 2018 Status Research & Development GmbH
+# Copyright (c) 2018-2020 Status Research & Development GmbH
 # Licensed and distributed under either of
 #   * MIT license (license terms in the root directory or at https://opensource.org/licenses/MIT).
 #   * Apache v2 license (license terms in the root directory or at https://www.apache.org/licenses/LICENSE-2.0).
@@ -7,11 +7,11 @@
 
 import
   # Standard library
-  os,
+  os, tables,
   # Status libraries
   confutils/defs, serialization,
   # Beacon-chain
-  ../beacon_chain/spec/[datatypes, crypto, beaconstate, validator, state_transition_block, state_transition_epoch],
+  ../beacon_chain/spec/[datatypes, crypto, helpers, beaconstate, validator, state_transition_block, state_transition_epoch],
   ../beacon_chain/[ssz, state_transition, extras]
 
 # Nimbus Bench - Scenario configuration
@@ -185,6 +185,9 @@ template processEpochScenarioImpl(
 
   when needCache:
     var cache = get_empty_per_epoch_cache()
+    let epoch = state.data.slot.compute_epoch_at_slot
+    cache.shuffled_active_validator_indices[epoch] =
+      get_shuffled_active_validator_indices(state.data, epoch)
 
   # Epoch transitions can't fail (TODO is this true?)
   when needCache:
@@ -251,11 +254,11 @@ genProcessEpochScenario(runProcessJustificationFinalization,
 
 genProcessEpochScenario(runProcessRegistryUpdates,
                         process_registry_updates,
-                        needCache = false)
+                        needCache = true)
 
 genProcessEpochScenario(runProcessSlashings,
                         process_slashings,
-                        needCache = false)
+                        needCache = true)
 
 genProcessEpochScenario(runProcessFinalUpdates,
                         process_final_updates,
diff --git a/tests/official/test_fixture_state_transition_epoch.nim b/tests/official/test_fixture_state_transition_epoch.nim
index 6f9cf1096..4bb94652e 100644
--- a/tests/official/test_fixture_state_transition_epoch.nim
+++ b/tests/official/test_fixture_state_transition_epoch.nim
@@ -66,13 +66,13 @@ runSuite(JustificationFinalizationDir, "Justification & Finalization",  process_
 # ---------------------------------------------------------------
 
 const RegistryUpdatesDir = SszTestsDir/const_preset/"phase0"/"epoch_processing"/"registry_updates"/"pyspec_tests"
-runSuite(RegistryUpdatesDir, "Registry updates",  process_registry_updates, useCache = false)
+runSuite(RegistryUpdatesDir, "Registry updates",  process_registry_updates, useCache = true)
 
 # Slashings
 # ---------------------------------------------------------------
 
 const SlashingsDir = SszTestsDir/const_preset/"phase0"/"epoch_processing"/"slashings"/"pyspec_tests"
-runSuite(SlashingsDir, "Slashings",  process_slashings, useCache = false)
+runSuite(SlashingsDir, "Slashings",  process_slashings, useCache = true)
 
 # Final updates
 # ---------------------------------------------------------------
diff --git a/tests/spec_epoch_processing/justification_finalization_helpers.nim b/tests/spec_epoch_processing/justification_finalization_helpers.nim
index b0a7e04ae..5bce78436 100644
--- a/tests/spec_epoch_processing/justification_finalization_helpers.nim
+++ b/tests/spec_epoch_processing/justification_finalization_helpers.nim
@@ -7,7 +7,7 @@
 
 import
   # Standard library
-  strformat,
+  strformat, tables,
   # Specs
   ../../beacon_chain/spec/[datatypes, state_transition_epoch, validator, helpers],
   # Test helpers
@@ -34,7 +34,10 @@ proc addMockAttestations*(
     raise newException(ValueError, &"Cannot include attestations from epoch {state.get_current_epoch()} in epoch {epoch}")
 
   # TODO: Working with an unsigned Gwei balance is a recipe for underflows to happen
-  var remaining_balance = state.get_total_active_balance().int64 * 2 div 3
+  var cache = get_empty_per_epoch_cache()
+  cache.shuffled_active_validator_indices[epoch] =
+    get_shuffled_active_validator_indices(state, epoch)
+  var remaining_balance = state.get_total_active_balance(cache).int64 * 2 div 3
 
   let start_slot = compute_start_slot_at_epoch(epoch)