After trying out the IE9 beta for a while, I will have to disagree with you. Performance is really nice, especially coming from Firefox which is getting slower and slower with every update and by now feels like it spends more time freezing than responding. IE9 has definitely surpassed Firefox in terms of performance, and is on par with the other major browsers out there.

As for HTML5 support, I don't know the particulars, but IE9 scores 95/100 in the ACID3 tests, so it can't be that bad. And it's still beta, so maybe it will reach 100 in the final release (IE8 had 17/100). Besides, other browsers have their own "convenience extensions" to HTML and CSS, Webkit and Opera being prime examples. As long as standards-compliant content is rendered appropriately, I don't see the problem, and so far I haven't had any problems with HTLM5/CSS3 rendering in IE9.