bumpski - 4:57 pm on Jan 12, 2011 (gmt 0)
Ads and specifically Adsense ads can really kill pageload time as reported by webmaster tools pageload. Also Google Analytics has a significant negative impact.
Firefox is an excellent resource (Firebug, pageload), but try using an ad blocker and your pageload times will drop dramatically for your own browser.
If you use an ad blocker, enable PageRank in the Google Toolbar, and surf your own site it is pretty easy to significantly influence the Webmaster Tools performance measure. Webmaster Tools "pageload" purposely filters out indications of slowness generated by Adsense Ads and Google Analytics. Only the Firefox Pageload tool shows whats really going on!
It comes down to how many visitors to your site are actually using the Google Toolbar with PageRank enabled, probably not many. You may be the one providing Google with data that says your site is slow!
I wouldn't be surprised if soon Google starts using its page preview functionality to measure total pageload performance. If they do I hope they speed up Adsense at the same time.