doc: add performance note for ErrorisError deprecation

Author: miguelmarcondesf

doc/api/util.md

@@ -3324,6 +3324,9 @@ deprecated: REPLACEME
 
 > Stability: 0 - Deprecated: Use [`Error.isError`][] instead.
 
+**Note:** As of Node.js v24, `Error.isError()` is currently slower than `util.types.isNativeError()`.
+If performance is critical, consider benchmarking both in your environment.
+
 * `value` {any}
 * Returns: {boolean}