I just grabbed a fix from WebKit for a javascript bug (
http://bugzilla.opendarwin.org/show_bug.cgi?id=9031) that ought to help with the slowdowns you have been seeing as well as 1/3 of our crash reports and our memory usage.
The problem was that javascript can start up timed events and they never stop if you close the window (or possibly when browsing to a new page either) thus keeping the DOM objects around (affecting memory) and slowing down the application using unnecessary CPU time.
I am especially happy since I also reported the bug in the first place. :)