Order by id instead of inserted_at to better use indexes.

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -119,7 +119,7 @@ defp restrict_actor(query, _), do: query
   def fetch_activities(recipients, opts \\ %{}) do
     base_query = from activity in Activity,
       limit: 20,
-      order_by: [desc: :inserted_at]
+      order_by: [desc: :id]
 
     base_query
     |> restrict_recipients(recipients)