If, like me, you hate it when your MacBook's fans start to whine while watching YouTube, you should skip Safari (even 4.0 Beta) and try OmniWeb (now a free product) or Camino instead. A comparison of the Activity Monitor results, using the same video in each browser, showed these figures:
Safari 4.0 Beta
Safari 50%
Window Server 12%
OmniWeb 5.9
OmniWeb 36%
Window Server 5%
Camino 1.6
Camino 33%
Window Server 5%
Here are some screen grabs showing each browser's performance. Further remarkable is the fact that Camino split the workload across the cores nearly evenly. I tested on a Macbook 1.83GHz Core 2 Duo with 2GB of RAM running OS X 10.4.11.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090302135816608