docs(readme): feature drawback

Author: KSXGitHub

README.md

@@ -50,7 +50,8 @@ The benchmark was generated by [a GitHub Workflow](https://github.com/KSXGitHub/
 * Very fast.
 * Relative comparison of separate files.
 * Extensible via the library crate or JSON interface.
-* Optional progress report (would make `pdu` slightly slower).
+* Optional progress report:
+  - Drawback: It would make `pdu` slightly slower.
 * Customize tree depth.
 * Customize chart size.