tedster - 9:07 pm on Jan 11, 2011 (gmt 0)
A lot of the factors that impact Site Performance are about page rendering, not server response time.
In my experience, things like how many http calls need to be made by the browser (scripts, styles images) how well optimized the images are, and the use of gzip compression are extremely critical compared to a few hundred ms improvement in server response time.
Google may report this to you in one interface, but they have more than one "tool" here. Probably the most important source of information about your pages comes from real visitors who have the toolbar installed. It phones home and tells tales.
I'm not saying that the Site Performance numbers are not mysterious at times - but I am saying we need to look at the whole performance picture, and not only server factors.