Do not calculate presence or ephemeral events when they are filtered out (#14970)

This expands the previous optimisation from being only for initial
sync to being for all sync requests.

It also inverts some of the logic to be inclusive instead of exclusive.

changelog.d/14970.misc

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

synapse/handlers/sync.py

@@ -1459,10 +1459,12 @@ class SyncHandler:
             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 not filtered out.
+        include_presence_data = (
+            self.hs_config.server.use_presence
+            and 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,
@@ -1841,15 +1843,12 @@ class SyncHandler:
         """
 
         since_token = sync_result_builder.since_token
+        user_id = sync_result_builder.sync_config.user.to_string()
 
         # 1. Start by fetching all ephemeral events in rooms we've joined (if required).
-        user_id = sync_result_builder.sync_config.user.to_string()
-        block_all_room_ephemeral = (
-            since_token is None
-            and sync_result_builder.sync_config.filter_collection.blocks_all_room_ephemeral()
-        )
-
-        if block_all_room_ephemeral:
+        if (
+            sync_result_builder.sync_config.filter_collection.blocks_all_room_ephemeral()
+        ):
             ephemeral_by_room: Dict[str, List[JsonDict]] = {}
         else:
             now_token, ephemeral_by_room = await self.ephemeral_by_room(