setzer - 8:36 pm on Jun 6, 2010 (gmt 0)
A ping will not give you browser load speed, but only server/network response time - and Google's metric is the time for the page to fully render in a user's browser.
indeed, I realize this. I've used other services which calculate the render time in browser, including external factors such as iframes and third-party scripts. The total render time is only about 0.5 secs higher and that is mainly due to advertisements that load up in an iframe.
I think londrum's belief that WMT is reporting on old data may have merit. Over the past few days, our site has gone from 60% to 68% slower and no changes have been made during this time.