Measuring web page load time is a notoriously tricky but important endeavor. One of the most common challenges is simply getting a true start time. Historically, the earliest a web page could reliably begin measurement is when the browser begins to parse an HTML document.
Unfortunately, that is too late to include a significant portion of the time web surfers spend waiting for the page: much of the time is spent fetching the page from the web server. To address this shortcoming, some clever web developers work around the problem by storing the navigation start time in a cookie during the previous page’s onbeforeunload handler. However, this doesn’t work for the critical first page load which likely has a cold cache.
Web Timing now gives developers the ability to measure the true page load time by including the time to request, generate, and receive the HTML document. The timeline below illustrates the metrics it provides. The vertical line labeled "Legacy navigation started" is the earliest time a web page can reliably measure without Web Timing. In this case, instead of a misleading 80ms load time, it is now possible to see that the user actually experienced a 274ms time. Including this missing phase will make your measurements appear to increase. It’s not because pages are getting slower – we’re just getting a better view on where the time is actually being spent.
Across other browsers: Web Timing metrics are under window.msPerformance in the third platform preview of Internet Explorer 9 and work is underway to add window.mozPerformance to Firefox. The specification is still being finalized, so expect slight changes before the browser prefixes are dropped.