bumpski - 12:19 pm on Dec 17, 2010 (gmt 0)
There are probably so few people surfing our sites with the Google Toolbar and PageRank turned on that the performance data can be influenced. In fact it may be the webmasters themselves feeding Google the poor performance data. I frequently tour my sites with tons of apps running and frequently with the PageRank stat on, delaying the Window onload event substantially.
What's truly annoying was the first step Google took to solve this problem was to remove any sign of the poor Adsense and Analytics performance from the Webmaster Tools Labs report. If you use Firefox and the actual Pageload speed tool, you'll see all the problems.
Finally turning off image ads may produce enough of a pageload performance boost that reduced load time might increase earnings more than the additional image ad bidders. Plus image ads are far more likely to be CPM, so for lower traffic, high content sites, these CPM ads degrade earnings by displacing the better paying CPC ads. Google has not figured out how to calculate the true "effective CPM" for CPC ads on lower traffic pages(Another topic!).