As for performance, you're right that if the render time was on the order to milliseconds, most users don't notice. The problem is that it typically isn't. Considering that we're most likely lazy-loading CSS/HTML/XML/etc, we have to wait for the disk access time, the parsing time, the layout engine processing time, and then the admittedly miniscule drawing time. This adds up both in latency, cpu, and memory usage.
Consider Safari, which I'm guessing Apple has gone to no small lengths to improve the performance of. 6 seconds to load gmail on my dual G4. It's about the same amount of time necessary to load and check mail using Microsoft Internet Mail and News on a old LC575 I think.
Web browsers have gotten bloated to the point where simply launching one allocates more memory than a old computer running Netscape 3 had in both ram and disk together. There is little that could be done better with AJAX that couldn't have been done with a 10 year old Mac running Netscape 3 and Flash.
Now, sure, it might not feel so bad on your desktop with two processors. But a handheld device can't afford to run a text interpreter when an app wants to draw a button, for each button.