diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index f2ff4e5a18..9da47a0b0e 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -686,7 +686,7 @@ class EventCreationHandler:
                 )
                 return (
                     prev_event,
-                    await self.store.get_stream_id_for_event(prev_event.event_id),
+                    prev_event.internal_metadata.stream_ordering,
                 )
 
         return await self.handle_new_client_event(
diff --git a/synapse/storage/persist_events.py b/synapse/storage/persist_events.py
index 97e63a1dfc..d656957942 100644
--- a/synapse/storage/persist_events.py
+++ b/synapse/storage/persist_events.py
@@ -285,6 +285,14 @@ class EventsPersistenceStorage:
         return event, pos, self.main_store.get_room_max_token()
 
     def _maybe_start_persisting(self, room_id: str):
+        """Pokes the `_event_persist_queue` to start handling new items in the
+        queue, if not already in progress.
+
+        Causes the deferreds returned by `add_to_queue` to resolve with: a
+        dictionary of event ID to event ID we didn't persist as we already had
+        another event persisted with the same TXN ID.
+        """
+
         async def persisting_queue(item):
             with Measure(self._clock, "persist_events"):
                 return await self._persist_events(