I ran across an awesome presentation by Paul Irish discussing DOM and CSS3 performance issues:

The video: http://www.youtube.com/watch?v=q_O9_C2ZjoA
The slides: http://dl.dropbox.com/u/39519/talks/gperf/index.html
The blog post: http://paulirish.com/2011/dom-html5-css3-performance/

The top tips that were new to me:

• how to not trigger unnecessary layout/reflow
• iOS Safari: get massive compositing performance improvement by using CSS3’s translate3d(0,0,0) instead of translate(0,0)
• jsperf.com to conduct JavaScript profiling experiments: example