diff --git a/federationapi/routing/eventauth.go b/federationapi/routing/eventauth.go
index dab17f084..003165c85 100644
--- a/federationapi/routing/eventauth.go
+++ b/federationapi/routing/eventauth.go
@@ -29,6 +29,8 @@ func GetEventAuth(
 	roomID string,
 	eventID string,
 ) util.JSONResponse {
+	// TODO: Optimisation: we shouldn't be querying all the room state
+	// that is in state.StateEvents - we just ignore it.
 	state, err := getState(ctx, request, query, roomID, eventID)
 	if err != nil {
 		return *err