Google recently added ‘Site Performance’ tab under Labs in Webmaster Tool. This new application will show you the page load time for added sites in webmaster tools. Based on the load time from different DNS lookups and other objects available on your site, Google determines the page load time on different connections and then it takes the average load time and show you under this tab.
Google is also promoting its own application ‘Page Speed’ which really helps in optimizing the pages for reduced load time. This is similar to YSlow add-on for Firefox which is an application from Yahoo. You can optimize your site using YSlow as well which is much better than Page Speed (right now). Google may improve Page Sped in coming days.
After optimizing your pages, you may wait for couple of days to get the latest average load time from Google under ‘Site Performance’ lab.