A <i>cross-browser</i> solution for the same problem is to use the Fiddler2 Proxy/API to collect the performance Data. AlertFox uses this approach:
<a href="http://blog.alertfox.com/2011/05/upcoming-features-web-traffic-protocols.html" rel="nofollow">http://blog.alertfox.com/2011/05/upcoming-features-web-traff...</a>