Several times I have gone looking for an article like this on Google, and several times I have failed. Tonight I hit just the right combination of keywords, and up popped this article straight from the horses mouth:
Whenever I'm writing rules for CSS, I always find myself asking questions like "Is it faster for the client rendering engine if I use #id as a selector, or if I use div#id"? This article answers that and many other questions - and exposes several things I've been doing all wrong, thinking I was actually improving the render speed.
BTW, from the article:
BAD - tag#id
GOOD - #id"