My trading platform, tradeking.com, has a web app called Live Trading. According to the task manager, it maxes out 1 of my cores, and makes Firefox's UI unusable in both my home and work computers. My solution has been to run that app in Chrome, and do my light duty browsing in FF. Chrome's CPU usage is also high, but the UI stays responsive.
Heh...it would be something I can't go to. It sounds more like inefficient JS than any HTML5 issues, but that is one drawback to not separating tabs; if the JS on one is maxing a core, you can't do anything anywhere else.