diff --git a/changelog.d/13586.misc b/changelog.d/13586.misc
new file mode 100644
index 0000000000..daa71470f6
--- /dev/null
+++ b/changelog.d/13586.misc
@@ -0,0 +1 @@
+Comment about a better future where we can get the state diff between two events.
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index 048c4111f6..ace7adcffb 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -1041,6 +1041,14 @@ class FederationEventHandler:
             InvalidResponseError: if the remote homeserver's response contains fields
                 of the wrong type.
         """
+
+        # It would be better if we could query the difference from our known
+        # state to the given `event_id` so the sending server doesn't have to
+        # send as much and we don't have to process as many events. For example
+        # in a room like #matrix:matrix.org, we get 200k events (77k state_events, 122k
+        # auth_events) from this call.
+        #
+        # Tracked by https://github.com/matrix-org/synapse/issues/13618
         (
             state_event_ids,
             auth_event_ids,