diff --git a/changelog.d/6494.bugfix b/changelog.d/6494.bugfix
new file mode 100644
index 0000000000..78726d5d7f
--- /dev/null
+++ b/changelog.d/6494.bugfix
@@ -0,0 +1 @@
+Back out cross-signing code added in Synapse 1.5.0, which caused a performance regression.
diff --git a/synapse/handlers/e2e_keys.py b/synapse/handlers/e2e_keys.py
index 28c12753c1..57a10daefd 100644
--- a/synapse/handlers/e2e_keys.py
+++ b/synapse/handlers/e2e_keys.py
@@ -264,7 +264,6 @@ class E2eKeysHandler(object):
 
         return ret
 
-    @defer.inlineCallbacks
     def get_cross_signing_keys_from_cache(self, query, from_user_id):
         """Get cross-signing keys for users from the database
 
@@ -284,35 +283,14 @@ class E2eKeysHandler(object):
         self_signing_keys = {}
         user_signing_keys = {}
 
-        for user_id in query:
-            # XXX: consider changing the store functions to allow querying
-            # multiple users simultaneously.
-            key = yield self.store.get_e2e_cross_signing_key(
-                user_id, "master", from_user_id
-            )
-            if key:
-                master_keys[user_id] = key
-
-            key = yield self.store.get_e2e_cross_signing_key(
-                user_id, "self_signing", from_user_id
-            )
-            if key:
-                self_signing_keys[user_id] = key
-
-            # users can see other users' master and self-signing keys, but can
-            # only see their own user-signing keys
-            if from_user_id == user_id:
-                key = yield self.store.get_e2e_cross_signing_key(
-                    user_id, "user_signing", from_user_id
-                )
-                if key:
-                    user_signing_keys[user_id] = key
-
-        return {
-            "master_keys": master_keys,
-            "self_signing_keys": self_signing_keys,
-            "user_signing_keys": user_signing_keys,
-        }
+        # Currently a stub, implementation coming in https://github.com/matrix-org/synapse/pull/6486
+        return defer.succeed(
+            {
+                "master_keys": master_keys,
+                "self_signing_keys": self_signing_keys,
+                "user_signing_keys": user_signing_keys,
+            }
+        )
 
     @trace
     @defer.inlineCallbacks
diff --git a/sytest-blacklist b/sytest-blacklist
index 411cce0692..79b2d4402a 100644
--- a/sytest-blacklist
+++ b/sytest-blacklist
@@ -33,3 +33,6 @@ New federated private chats get full presence information (SYN-115)
 # Blacklisted due to https://github.com/matrix-org/matrix-doc/pull/2314 removing
 # this requirement from the spec
 Inbound federation of state requires event_id as a mandatory paramater
+
+# Blacklisted until https://github.com/matrix-org/synapse/pull/6486 lands
+Can upload self-signing keys
diff --git a/tests/handlers/test_e2e_keys.py b/tests/handlers/test_e2e_keys.py
index 854eb6c024..fdfa2cbbc4 100644
--- a/tests/handlers/test_e2e_keys.py
+++ b/tests/handlers/test_e2e_keys.py
@@ -183,6 +183,10 @@ class E2eKeysHandlerTestCase(unittest.TestCase):
         )
         self.assertDictEqual(devices["master_keys"], {local_user: keys2["master_key"]})
 
+    test_replace_master_key.skip = (
+        "Disabled waiting on #https://github.com/matrix-org/synapse/pull/6486"
+    )
+
     @defer.inlineCallbacks
     def test_reupload_signatures(self):
         """re-uploading a signature should not fail"""
@@ -503,3 +507,7 @@ class E2eKeysHandlerTestCase(unittest.TestCase):
             ],
             other_master_key["signatures"][local_user]["ed25519:" + usersigning_pubkey],
         )
+
+    test_upload_signatures.skip = (
+        "Disabled waiting on #https://github.com/matrix-org/synapse/pull/6486"
+    )