Skip soft fail checks for rooms with partial state

changelog.d/13354.misc

@@ -0,0 +1 @@
+Faster room joins: skip soft fail checks while Synapse only has partial room state, since the current membership of event senders may not be accurately known.

synapse/handlers/federation_event.py

@@ -1664,11 +1664,21 @@ async def _check_for_soft_fail(
         """Checks if we should soft fail the event; if so, marks the event as
         such.
 
+        Does nothing for events in rooms with partial state, since we may not have an
+        accurate membership event for the sender in the current state.
+
         Args:
             event
             state_ids: The state at the event if we don't have all the event's prev events
             origin: The host the event originates from.
         """
+        if await self._store.is_partial_state_room(event.room_id):
+            # We might not know the sender's membership in the current state, so don't
+            # soft fail anything. Even if we do have a membership for the sender in the
+            # current state, it may have been derived from state resolution between
+            # partial and full state and may not be accurate.
+            return
+
         extrem_ids_list = await self._store.get_latest_event_ids_in_room(event.room_id)
         extrem_ids = set(extrem_ids_list)
         prev_event_ids = set(event.prev_event_ids())