add parallel execution note to mapreduce

docs/design_pattern/mapreduce.md

@@ -68,3 +68,6 @@ flow.run(shared)
 print("Individual Summaries:", shared["file_summaries"])
 print("\nFinal Summary:\n", shared["all_files_summary"])
 ```
+
+> **Performance Tip**: The example above works sequentially. You can speed up the map phase by running it in parallel. See [(Advanced) Parallel](../core_abstraction/parallel.md) for more details.
+{: .note }