This is very likely how Google measures site speed for ranking, as well as anonymous submissions from the Google Toolbar. Google Analytics also lets you filter this data by other metrics such as browser and geographic location.
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.
Now we have previews in the search results, so we know for sure that Google renders websites.
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.
So while Google does this with a HTML5 feature in modern browsers, internally it should already have:
- page speed scores
- render scores (perhaps render time)
of all websites in index, regardless the browser their users use.
That is how I think Google measures site speed; it uses these known non-HTML5 factors for ranking the fastest websites.