Do not calculate presence or ephemeral events when they are filtered out

changelog.d/14970.misc

@@ -0,0 +1 @@
+Improve performance of `/sync` in a few situations.

synapse/handlers/sync.py

@@ -1459,10 +1459,14 @@ async def generate_sync_result(
             sync_result_builder, account_data_by_room
         )
 
-        block_all_presence_data = (
-            since_token is None and sync_config.filter_collection.blocks_all_presence()
+        # Presence data is included if the server has it enabled and:
+        # - There is a sync token, or
+        # - Is not filtered out.
+        include_presence_data = self.hs_config.server.use_presence and (
+            since_token is not None
+            or not sync_config.filter_collection.blocks_all_presence()
         )
-        if self.hs_config.server.use_presence and not block_all_presence_data:
+        if include_presence_data:
             logger.debug("Fetching presence data")
             await self._generate_sync_entry_for_presence(
                 sync_result_builder,