Don't schedule an async task on every sync (#16312)

changelog.d/16312.misc

@@ -0,0 +1 @@
+Delete device messages asynchronously and in staged batches using the task scheduler.

synapse/handlers/sync.py

@@ -362,7 +362,22 @@ class SyncHandler:
         # (since we now know that the device has received them)
         if since_token is not None:
             since_stream_id = since_token.to_device_key
-            # Delete device messages asynchronously and in batches using the task scheduler
+            # Fast path: delete a limited number of to-device messages up front.
+            # We do this to avoid the overhead of scheduling a task for every
+            # sync.
+            device_deletion_limit = 100
+            deleted = await self.store.delete_messages_for_device(
+                sync_config.user.to_string(),
+                sync_config.device_id,
+                since_stream_id,
+                limit=device_deletion_limit,
+            )
+            logger.debug(
+                "Deleted %d to-device messages up to %d", deleted, since_stream_id
+            )
+
+            # If we hit the limit, schedule a background task to delete the rest.
+            if deleted >= device_deletion_limit:
                 await self._task_scheduler.schedule_task(
                     DELETE_DEVICE_MSGS_TASK_NAME,
                     resource_id=sync_config.device_id,