Web Performance Daybook Volume 2

Chapter 19. Stop Wasting Your Time Using the Google Analytics Site Speed Report

Aaron Peters

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.

Problem: A Bug in Firefox Implementation of the Navigation Timing API

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 = ...