Ken at Omni Group just posted a quick note in this MacNN forum thread to let everyone know that OmniWeb's UI is much slower than expected when using the Graphite OS X theme. Switching to the Aqua theme will resolve the issue in the current release, and Ken writes that he's fixed the bug in the next release.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010810190211193