Search and install the plugin using the Extension Manager. Refer to Plugins on how to install plugins manually.

Stopwatch plugin measures execution time of PHP code on parts of the page. Usage:

<stopwatch reset>

{token_which_takes_much_processing_time}

Time elapsed: <stopwatch show>

This will display time consumed by {token_which_takes_much_processing_time} in seconds.

See the plugin in action here.