I'm continuing to do more research into issue 193, which is currently our largest performance issue (the URL I use for testing is local radio station KFI). I'm beginning to suspect the problem is Apple's atrocious pthreads implementation. Is it as bad on 10.5 as it is on 10.4? Your comments suggested. If it's really just this bad, it may not be fixable without Mozilla doing a deep dive on the layout code, which I bet they won't.
17.0.3 will have issue 205 also since it works very well, plus the Spanish and Russian langpacks, plus Claudio's Intel patches. Look for that over the weekend for release on Monday.