Scoble knocks IE7's performance compared to Firefox 2 and I had to see it to believe it. Given I've been living in IE7 for months - and more recently with IE7 on Windows Vista - I've never given performance a thought, which tells me either that performance has been fine or I'm simply ignorant. Likely a little of both. But I wonder, does it really matter?
Even so, I tried Robert's test (and that meant actually downloading Firefox) by setting the home pages for both browsers to http://www.techmeme.com and sure enough, there's a noticeable difference in overall rendering speed. Firefox's rendering makes the page just "pop" whereas IE7 spends some visible time tweaking the page as it downloads. It's almost as if IE7 is not grabbing the image files from cache.
In fact, I tried the same test with sandbox.netfx3.com and see the same rendering behavior by both browsers on the first visit. But on subsequent visits, Firefox again makes the page "pop" where as IE7 appears to be downloading the content.
As for his beef about AJAX performance, on my 64-bit Windows Vista RC2 machine, I do not see any difference in the experience between IE7 and Firefox like he describes using Google Maps.