Extremely slow scrolling on Apple trackpads

I'm seeing a scrolling performance problem in Qt 4.8 RC1. For moderately complex webpages, QWebView and QGraphicsWebView are exhibiting very slow scrolling with Apple trackpads (I've tested so far on the internal one built into my Mid-2011 MacBook Pro, as well as a Magic Trackpad). But, when the window is deactivated, the scrolling performance issue basically goes away. When the window is reactivated then the problem is back.

I'm attaching a video of the problem as seen with the Fancybrowser example app, viewing the Google News homepage.

This is a big problem for our product (http://www.rstudio.org) as we host a quite complex HTML-based UI in QtWebkit and in some cases the scrolling is unusably bad. (Again, only when activated.)

I see that this problem or something like it has been reported in the past for Qt 4.7 versions:

QTBUG-16684
QTBUG-15869

I don't see this problem in Qt 4.7.3 but I do in Qt 4.8 RC1.

Thanks!