The referenced page is rather slow when scrolling or resizing (maxes out my Core 2). Shark indicates that around 50% of the time is spent drawing shadows.
The page has a static background, so it does a full repaint on scroll. With bug 13320 fixed, I do not see anything obvious that can be improved on the WebCore side.
This actually seems way better in TOT than in shipping Safari on 10.5.5. I am tempted to say this is fixed, but I would like a second opinion. Dan, what do you think?
Much better in WebKit nightly r42583.