Avoid walking script variable ChainMap twice when tracing (#173665)

homeassistant/helpers/trace.py

@@ -87,13 +87,15 @@ class TraceElement:
         if variables is None:
             variables = {}
         last_variables = self._last_variables
-        variables_cv.set(dict(variables))
-        changed_variables = {
+        # variables is often a ChainMap which is costly to iterate, so flatten
+        # it once and reuse the snapshot for both the baseline and the diff.
+        snapshot = dict(variables)
+        variables_cv.set(snapshot)
+        self._variables = {
             key: value
-            for key, value in variables.items()
+            for key, value in snapshot.items()
             if key not in last_variables or last_variables[key] != value
         }
-        self._variables = changed_variables
 
     def as_dict(self) -> dict[str, Any]:
         """Return dictionary version of this TraceElement."""