Reduce DB load when forget on leave setting is disabled (#16668)

* Reduce DB load when forget on leave setting is disabled

* Newsfile

changelog.d/16668.misc

@@ -0,0 +1 @@
+Reduce DB load when forget on leave setting is disabled.

synapse/handlers/room_member.py

@@ -2111,9 +2111,14 @@ async def _unsafe_process(self) -> None:
                 self.pos = room_max_stream_ordering
 
         if not self._hs.config.room.forget_on_leave:
-            # Update the processing position, so that if the server admin turns the
-            # feature on at a later date, we don't decide to forget every room that
-            # has ever been left in the past.
+            # Update the processing position, so that if the server admin turns
+            # the feature on at a later date, we don't decide to forget every
+            # room that has ever been left in the past.
+            #
+            # We wait for a short time so that we don't "tight" loop just
+            # keeping the table up to date.
+            await self._clock.sleep(0.5)
+
             self.pos = self._store.get_room_max_stream_ordering()
             await self._store.update_room_forgetter_stream_pos(self.pos)
             return