I'm getting this error as well, making posts on The Matrix Online forums:
http://forums.station.sony.com/mxo/forums/list.m
If I select editing options in the first few seconds of loading the page, I can insert images, edit text, and do virtually whatever I need to make my post. Any time after that, and the buttons stop being responsive. If I try to type my message without formatting, I can do so, but I get the slow script error every few seconds.
I get none of these errors when using Firefox on the same page, and although the buttons were also broken in safari 2, at least with that version I could access an HTML version of the editor, so I could apply formatting that way.
Safari 3 has a lot of great features, but this slow script error is driving me nuts, and I can't figure out how to disable its constant nagging.
I am currently using Safari 3.0.4 with Tiger 10.4.11.