You might want to check out FJ's weblog, "Homebrew Safari."
http://macdevcenter.com/pub/wlg/7651
He's posted an AppleScript that basically cleans every corner of Safari's house. This might do the trick for many folks experiencing sluggishness with the browser.
I haven't run FJ's script yet... I'm going to watch the talkbacks for a day or so first :)