A Google Webmaster Help thread asks why the site performance report in Google Webmaster Tools hasn't updated in 15 days.
The last time the report updated for this user was on September 1, 2010. The individual said they "updated our site last night with speed enhancements (now 2 to 3 times faster than previous)" and wants to know when to expect Google to update the report.
Well, it isn't just an issue for him. I checked ~30% of the sites I have access to in Google Webmaster Tools (and I have a lot) and they all said September 1st. The brackets say "updated on Sep 1, 2010."
I should note that the site performance is currently a "labs" project and is allowed to have problems and be slow to update.
Forum discussion at Google Webmaster Help.