diff --git a/synapse/storage/event_federation.py b/synapse/storage/event_federation.py
index f0aa2193fb..ee88c61954 100644
--- a/synapse/storage/event_federation.py
+++ b/synapse/storage/event_federation.py
@@ -129,7 +129,7 @@ class EventFederationStore(SQLBaseStore):
             room_id,
         )
 
-    @cached()
+    @cached(max_entries=5000, iterable=True)
     def get_latest_event_ids_in_room(self, room_id):
         return self._simple_select_onecol(
             table="event_forward_extremities",