How does Google determine page speed?

How does Google determine page speed? In GWT some pages are listed as very slow (8+ seconds). But I have tested on older computers/browsers and they do not take anywhere near that long to load. Why might Google show such high numbers? DisgruntledGoat, UK Have a question? Ask it in our Webmaster Help Forum: www.google.com Want your question to be answered on a video like this? Follow us on Twitter and look for an announcement when we take new questions: twitter.com More videos: www.youtube.com Webmaster Central Blog: googlewebmastercentral.blogspot.com Webmaster Central: www.google.com