In reply to the title of your question, "Site performance" in Google Webmaster Tools is measured as "the total time from the moment the user clicks on a link to your page until the time the entire page is loaded and displayed in a browser".
And it's collected from people that have the Google Toolbar installed, with the PageRank feature activated.
There is a tool, open sourced by Google, called Page Speed:
Page Speed is an open-source Firefox/Firebug Add-on. Webmasters and web developers can use Page Speed to evaluate the performance of their web pages and to get suggestions on how to improve them.
So you can use this tool, direct from Google, to measure Page Speed.