diff --git a/changelog.d/17789.misc b/changelog.d/17789.misc
new file mode 100644
index 0000000000..43ed360ce8
--- /dev/null
+++ b/changelog.d/17789.misc
@@ -0,0 +1 @@
+Speed up sliding sync when there are many active subscriptions.
diff --git a/synapse/handlers/sliding_sync/room_lists.py b/synapse/handlers/sliding_sync/room_lists.py
index 08e619042b..a1730b7e05 100644
--- a/synapse/handlers/sliding_sync/room_lists.py
+++ b/synapse/handlers/sliding_sync/room_lists.py
@@ -500,6 +500,16 @@ class SlidingSyncRoomLists:
                 # depending on the `required_state` requested (see below).
                 partial_state_rooms = await self.store.get_partial_rooms()
 
+                # Fetch any rooms that we have not already fetched from the database.
+                subscription_sliding_sync_rooms = (
+                    await self.store.get_sliding_sync_room_for_user_batch(
+                        user_id,
+                        sync_config.room_subscriptions.keys()
+                        - room_membership_for_user_map.keys(),
+                    )
+                )
+                room_membership_for_user_map.update(subscription_sliding_sync_rooms)
+
                 for (
                     room_id,
                     room_subscription,
@@ -507,17 +517,11 @@ class SlidingSyncRoomLists:
                     # Check if we have a membership for the room, but didn't pull it out
                     # above. This could be e.g. a leave that we don't pull out by
                     # default.
-                    current_room_entry = (
-                        await self.store.get_sliding_sync_room_for_user(
-                            user_id, room_id
-                        )
-                    )
+                    current_room_entry = room_membership_for_user_map.get(room_id)
                     if not current_room_entry:
                         # TODO: Handle rooms the user isn't in.
                         continue
 
-                    room_membership_for_user_map[room_id] = current_room_entry
-
                     all_rooms.add(room_id)
 
                     # Take the superset of the `RoomSyncConfig` for each room.
diff --git a/synapse/storage/databases/main/roommember.py b/synapse/storage/databases/main/roommember.py
index 0a62613d34..6f15e51339 100644
--- a/synapse/storage/databases/main/roommember.py
+++ b/synapse/storage/databases/main/roommember.py
@@ -1499,6 +1499,57 @@ class RoomMemberWorkerStore(EventsWorkerStore, CacheInvalidationWorkerStore):
             "get_sliding_sync_room_for_user", get_sliding_sync_room_for_user_txn
         )
 
+    async def get_sliding_sync_room_for_user_batch(
+        self, user_id: str, room_ids: StrCollection
+    ) -> Dict[str, RoomsForUserSlidingSync]:
+        """Get the sliding sync room entry for the given user and rooms."""
+
+        if not room_ids:
+            return {}
+
+        def get_sliding_sync_room_for_user_batch_txn(
+            txn: LoggingTransaction,
+        ) -> Dict[str, RoomsForUserSlidingSync]:
+            clause, args = make_in_list_sql_clause(
+                self.database_engine, "m.room_id", room_ids
+            )
+            sql = f"""
+                SELECT m.room_id, m.sender, m.membership, m.membership_event_id,
+                    r.room_version,
+                    m.event_instance_name, m.event_stream_ordering,
+                    m.has_known_state,
+                    COALESCE(j.room_type, m.room_type),
+                    COALESCE(j.is_encrypted, m.is_encrypted)
+                FROM sliding_sync_membership_snapshots AS m
+                INNER JOIN rooms AS r USING (room_id)
+                LEFT JOIN sliding_sync_joined_rooms AS j ON (j.room_id = m.room_id AND m.membership = 'join')
+                WHERE m.forgotten = 0
+                    AND {clause}
+                    AND user_id = ?
+            """
+            args.append(user_id)
+            txn.execute(sql, args)
+
+            return {
+                row[0]: RoomsForUserSlidingSync(
+                    room_id=row[0],
+                    sender=row[1],
+                    membership=row[2],
+                    event_id=row[3],
+                    room_version_id=row[4],
+                    event_pos=PersistedEventPosition(row[5], row[6]),
+                    has_known_state=bool(row[7]),
+                    room_type=row[8],
+                    is_encrypted=row[9],
+                )
+                for row in txn
+            }
+
+        return await self.db_pool.runInteraction(
+            "get_sliding_sync_room_for_user_batch",
+            get_sliding_sync_room_for_user_batch_txn,
+        )
+
 
 class RoomMemberBackgroundUpdateStore(SQLBaseStore):
     def __init__(