diff --git a/changelog.d/6954.misc b/changelog.d/6954.misc
new file mode 100644
index 000000000000..8b84ce2f1925
--- /dev/null
+++ b/changelog.d/6954.misc
@@ -0,0 +1 @@
+Minor perf fixes to `get_auth_chain_ids`.
diff --git a/synapse/storage/data_stores/main/event_federation.py b/synapse/storage/data_stores/main/event_federation.py
index 60c67457b4d1..d75ab0767580 100644
--- a/synapse/storage/data_stores/main/event_federation.py
+++ b/synapse/storage/data_stores/main/event_federation.py
@@ -15,7 +15,6 @@
 import itertools
 import logging
 
-from six.moves import range
 from six.moves.queue import Empty, PriorityQueue
 
 from twisted.internet import defer
@@ -27,6 +26,7 @@
 from synapse.storage.data_stores.main.signatures import SignatureWorkerStore
 from synapse.storage.database import Database
 from synapse.util.caches.descriptors import cached
+from synapse.util.iterutils import batch_iter
 
 logger = logging.getLogger(__name__)
 
@@ -71,14 +71,12 @@ def _get_auth_chain_ids_txn(self, txn, event_ids, include_given):
         front = set(event_ids)
         while front:
             new_front = set()
-            front_list = list(front)
-            chunks = [front_list[x : x + 100] for x in range(0, len(front), 100)]
-            for chunk in chunks:
+            for chunk in batch_iter(front, 100):
                 clause, args = make_in_list_sql_clause(
                     txn.database_engine, "event_id", chunk
                 )
-                txn.execute(base_sql + clause, list(args))
-                new_front.update([r[0] for r in txn])
+                txn.execute(base_sql + clause, args)
+                new_front.update(r[0] for r in txn)
 
             new_front -= results
 
diff --git a/synapse/storage/database.py b/synapse/storage/database.py
index 3eeb2f7c04fc..6dcb5c04dacf 100644
--- a/synapse/storage/database.py
+++ b/synapse/storage/database.py
@@ -1504,7 +1504,7 @@ def simple_search_list_txn(cls, txn, table, term, col, retcols):
 
 def make_in_list_sql_clause(
     database_engine, column: str, iterable: Iterable
-) -> Tuple[str, Iterable]:
+) -> Tuple[str, list]:
     """Returns an SQL clause that checks the given column is in the iterable.
 
     On SQLite this expands to `column IN (?, ?, ...)`, whereas on Postgres