How does Google Analytics measure site speed?

Since the announcement of the Page Speed Toolbar I predicted Google would not simple crawl a website, it would render it.<p>Now we have previews in the search results, so we know for sure that Google renders websites.<p>Next to detecting hidden spam or javascript layout changes, I presume this rendering can be used to produce a waterfall of the site loading process.<p>So while Google does this with a HTML5 feature in modern browsers, internally it should already have:<p>- page speed scores<p>- render scores (perhaps render time)<p>of all websites in index, regardless the browser their users use.<p>That is how I think Google measures site speed; it uses these known non-HTML5 factors for ranking the fastest websites.

I really wish GA tracked all the metrics exposed by the timing spec, instead of combining them into one overall value. It's great that this reports numbers from actual users' machines instead of a headless render process on a monitoring server somewhere though. Here is hoping FF implements support for this soon.