Here’s a quick overview of individual sections in Google Webmaster Tools' Site Performance feature:
- Performance overview shows a graph of aggregated speed numbers for website, based on pages that were most frequently accessed by visitors who use Google Toolbar with PageRank feature activated. If only a few visitors of your site use Google Toolbar, the data will not be shown in Webmaster Tools.
- Example pages with load times: you can find some example pages along with average, aggregated load times that users observed while they were on your website. These numbers may differ from what you see as they can come from a variety of different browsers, internet connections and locations. List can help you to recognize pages which take longer than average to load — pages that slow your users down. As page load times’re based on actual accesses made by your users, it's possible that it includes pages which are disallowed from crawling.
- Page Speed suggestions are based on Page Speed Firefox / Firebug plugin. In order to find details for these sample URLs, Google fetch page and all its embedded resources with Googlebot. If servers return slightly modified content for Googlebot than they would for normal users, this may affect what is shown here.
More info: Webmaster Central blog