Improve performance of generate diffs of state change events (#124601)

dict comps are inlined in cpython 3.12+

homeassistant/components/websocket_api/messages.py

@@ -224,9 +224,12 @@ def _state_diff_event(
     if (old_attributes := old_state.attributes) != (
         new_attributes := new_state.attributes
     ):
-        for key, value in new_attributes.items():
-            if old_attributes.get(key) != value:
-                additions.setdefault(COMPRESSED_STATE_ATTRIBUTES, {})[key] = value
+        if added := {
+            key: value
+            for key, value in new_attributes.items()
+            if key not in old_attributes or old_attributes[key] != value
+        }:
+            additions[COMPRESSED_STATE_ATTRIBUTES] = added
         if removed := old_attributes.keys() - new_attributes:
             # sets are not JSON serializable by default so we convert to list
             # here if there are any values to avoid jumping into the json_encoder_default