We know it's hard to measure performance. With the W3C, we're working to create an interoperable way to gather performance metrics about how long a webpage takes to load. This information is wired up into the Document Object Model (DOM) API and can be easily accessed once the page has loaded.
Try it out for yourself.
To find out more about the timings, just hover over the name for a quick description. Note that this interface is subject to change.