diff --git a/changelog.d/13153.misc b/changelog.d/13153.misc
new file mode 100644
index 0000000000..3bb51962e7
--- /dev/null
+++ b/changelog.d/13153.misc
@@ -0,0 +1 @@
+Reduce DB usage of `/sync` when a large number of unread messages have recently been sent in a room.
diff --git a/synapse/storage/databases/main/event_push_actions.py b/synapse/storage/databases/main/event_push_actions.py
index bb6e104d71..32536430aa 100644
--- a/synapse/storage/databases/main/event_push_actions.py
+++ b/synapse/storage/databases/main/event_push_actions.py
@@ -1013,8 +1013,7 @@ class EventPushActionsWorkerStore(ReceiptsWorkerStore, StreamWorkerStore, SQLBas
         sql = """
             SELECT user_id, room_id,
                 coalesce(old.%s, 0) + upd.cnt,
-                upd.stream_ordering,
-                old.user_id
+                upd.stream_ordering
             FROM (
                 SELECT user_id, room_id, count(*) as cnt,
                     max(stream_ordering) as stream_ordering
@@ -1042,7 +1041,6 @@ class EventPushActionsWorkerStore(ReceiptsWorkerStore, StreamWorkerStore, SQLBas
             summaries[(row[0], row[1])] = _EventPushSummary(
                 unread_count=row[2],
                 stream_ordering=row[3],
-                old_user_id=row[4],
                 notif_count=0,
             )
 
@@ -1063,57 +1061,27 @@ class EventPushActionsWorkerStore(ReceiptsWorkerStore, StreamWorkerStore, SQLBas
                 summaries[(row[0], row[1])] = _EventPushSummary(
                     unread_count=0,
                     stream_ordering=row[3],
-                    old_user_id=row[4],
                     notif_count=row[2],
                 )
 
         logger.info("Rotating notifications, handling %d rows", len(summaries))
 
-        # If the `old.user_id` above is NULL then we know there isn't already an
-        # entry in the table, so we simply insert it. Otherwise we update the
-        # existing table.
-        self.db_pool.simple_insert_many_txn(
+        self.db_pool.simple_upsert_many_txn(
             txn,
             table="event_push_summary",
-            keys=(
-                "user_id",
-                "room_id",
-                "notif_count",
-                "unread_count",
-                "stream_ordering",
-            ),
-            values=[
+            key_names=("user_id", "room_id"),
+            key_values=[(user_id, room_id) for user_id, room_id in summaries],
+            value_names=("notif_count", "unread_count", "stream_ordering"),
+            value_values=[
                 (
-                    user_id,
-                    room_id,
                     summary.notif_count,
                     summary.unread_count,
                     summary.stream_ordering,
                 )
-                for ((user_id, room_id), summary) in summaries.items()
-                if summary.old_user_id is None
+                for summary in summaries.values()
             ],
         )
 
-        txn.execute_batch(
-            """
-                UPDATE event_push_summary
-                SET notif_count = ?, unread_count = ?, stream_ordering = ?
-                WHERE user_id = ? AND room_id = ?
-            """,
-            (
-                (
-                    summary.notif_count,
-                    summary.unread_count,
-                    summary.stream_ordering,
-                    user_id,
-                    room_id,
-                )
-                for ((user_id, room_id), summary) in summaries.items()
-                if summary.old_user_id is not None
-            ),
-        )
-
         txn.execute(
             "UPDATE event_push_summary_stream_ordering SET stream_ordering = ?",
             (rotate_to_stream_ordering,),
@@ -1293,5 +1261,4 @@ class _EventPushSummary:
 
     unread_count: int
     stream_ordering: int
-    old_user_id: str
     notif_count: int