know how long it takes to load of your webpage, or site web?

The StopWatch can only measure websites that can be displayed in a frame. Some websites use javascript to break out of frames. This is not a StopWatch bug.
The stopwatch is a small Javascript that runs on your computer (not the Numion server). It measures the time between the moment your browser starts to load the URL and when the browser signals that it has finished ("Done" in the status bar). The measurement therefore includes fetching and interpreting all HTML (including frames), images, and Javascripts. It does not include content that is handled by plugins. If the page is in the cache then the stopwatch will measure the faster loading time (see tip above). The accuracy is the accuracy of the Javascript timer, on most systems 10 milliseconds.by TAATJENE