Fast-path filtering out all rooms.

synapse/api/filtering.py

@@ -283,6 +283,9 @@ async def filter_room_account_data(
             await self._room_filter.filter(events)
         )
 
+    def blocks_all_rooms(self) -> bool:
+        return self._room_filter.filters_all_rooms()
+
     def blocks_all_presence(self) -> bool:
         return (
             self._presence_filter.filters_all_types()

synapse/handlers/sync.py

@@ -1792,6 +1792,11 @@ async def _generate_sync_entry_for_rooms(
             - newly_left_rooms
             - newly_left_users
         """
+
+        # If the request doesn't care about rooms then nothing to do!
+        if sync_result_builder.sync_config.filter_collection.blocks_all_rooms():
+            return (set(), set(), set(), set())
+
         since_token = sync_result_builder.since_token
 
         # 1. Start by fetching all ephemeral events in rooms we've joined (if required).