After the iMac video plays the scrolling response up and down the page is terrible if the Quicktime plugin is scrolling in or out of the active view.
The bug url no longer contains a quicktime movie. However, going to: http://www.apple.com/trailers/universal/theexpress/large.html making the window small enough to need scrollbars, and scrolling around shows no performance problems. Tested with r35417 on 10.5.4 on a MacPro
Mass closing plug-in bugs, as plug-in support has been removed from WebKit. Please comment and/or reopen if this still affects WebKit in some way.