Since May 2011 the Site Speed report in Google Analytics shows how fast your pages load for your real visitors. Google Analytics measures page load time by using the Navigation Timing API (http://w3c-test.org/webperf/specs/NavigationTiming/) in all browsers that support it (IE9+, Chrome, FF7+, Android4+ (http://caniuse.com/#search=navigation)) and falls back to Google Toolbar data for older versions of IE and Firefox. Having page speed data in GA is great, because you can easily correlate it to bounce rate and conversion, resulting in great, actionable insight that down the road leads to a faster site, happier users, and more revenues. But if a significant percentage of your visitors use Firefox 7 or 8, you may very well be wasting a lot of time interpreting the Site Speed data and even more time taking the wrong actions.
Firefox implemented the Navigation Timing API in version 7, which
was released on September 27, 2011. From that day in that browser, there
has been a bug in the implementation of that API. You can read all about
it in the bug ticket (https://bugzilla.mozilla.org/show_bug.cgi?id=691547) on
Bugzilla. The problem is that the value for
window.performance.timing.navigationStart can be
too low, which means it is too far in the past. Google Analytics uses a
simple formula to calculate page load time:
loadTime = ...