diff --git a/changelog.d/11583.bugfix b/changelog.d/11583.bugfix
new file mode 100644
index 0000000000..d2ed113e21
--- /dev/null
+++ b/changelog.d/11583.bugfix
@@ -0,0 +1 @@
+Fix a performance regression in `/sync` handling, introduced in 1.49.0.
diff --git a/synapse/rest/client/sync.py b/synapse/rest/client/sync.py
index 88e4f5e063..7f5846d389 100644
--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@@ -522,7 +522,15 @@ class SyncRestServlet(RestServlet):
                 time_now=time_now,
                 # Don't bother to bundle aggregations if the timeline is unlimited,
                 # as clients will have all the necessary information.
-                bundle_aggregations=room.timeline.limited,
+                # bundle_aggregations=room.timeline.limited,
+                #
+                # richvdh 2021-12-15: disable this temporarily as it has too high an
+                # overhead for initialsyncs. We need to figure out a way that the
+                # bundling can be done *before* the events are stored in the
+                # SyncResponseCache so that this part can be synchronous.
+                #
+                # Ensure to re-enable the test at tests/rest/client/test_relations.py::RelationsTestCase.test_bundled_aggregations.
+                bundle_aggregations=False,
                 token_id=token_id,
                 event_format=event_formatter,
                 only_event_fields=only_fields,
diff --git a/tests/rest/client/test_relations.py b/tests/rest/client/test_relations.py
index 397c12c2a6..1b58b73136 100644
--- a/tests/rest/client/test_relations.py
+++ b/tests/rest/client/test_relations.py
@@ -574,11 +574,11 @@ class RelationsTestCase(unittest.HomeserverTestCase):
         assert_bundle(channel.json_body["event"]["unsigned"].get("m.relations"))
 
         # Request sync.
-        channel = self.make_request("GET", "/sync", access_token=self.user_token)
-        self.assertEquals(200, channel.code, channel.json_body)
-        room_timeline = channel.json_body["rooms"]["join"][self.room]["timeline"]
-        self.assertTrue(room_timeline["limited"])
-        _find_and_assert_event(room_timeline["events"])
+        # channel = self.make_request("GET", "/sync", access_token=self.user_token)
+        # self.assertEquals(200, channel.code, channel.json_body)
+        # room_timeline = channel.json_body["rooms"]["join"][self.room]["timeline"]
+        # self.assertTrue(room_timeline["limited"])
+        # _find_and_assert_event(room_timeline["events"])
 
         # Note that /relations is tested separately in test_aggregation_get_event_for_thread
         # since it needs different data configured.