Google’s fascination at making the web faster stretches on to the Webmaster Tools via Site Performance analytics feature. Site Performance which is currently under Webmaster Tools Labs provides you with various information that indicates how fast your site loads from the user’s end.Site Performance gives you the average page load time of the different pages comprising your websites or blogs. It also provides you with appropriate suggestions that you can do to improve problem areas of your sites loading performance.
I guess I'm not impressed because Alexa have had that metric for ages. I suppose Google are doing this because they are annoyed that slow sites are tying up their crawler.