weebmaster tools reports 10-20x grater response time than it actually is for one of the pages I maintain (haven't checked the others). So I would also conclude that it measures the time until every last CSS,JS and iframe is loaded. And that is just not right. Ok, I understand that they take CSS into account. But my pages work without JS, and I only use iframe-s to display ads. The point is that my page is readable long before JS files and ads are loaded.

I'd also like to know if they take caching of CSS,JS and image files into account.