(Sorry if I misassigned the component, but it's often hard for me to decide what's related to which component.) There seems to have been a regression since Moz0.8, which can be seen in the test pages given above. Scroll speed is WAY down in some of the test pages, almost to the point of not being usable. This was not a problem in Netscape 6.0/Mozilla 0.8 but has been a problem in the 0.9 family and recent nightly buids.
since this is not very affected by dhtml animations I would rather say a dupe of bug 87808
Marking this a duplicate of 100206, since that's a report by the author of the DHTML in question. If you want to dupe that report in reference to some other entry, fine, but it's a much better place to discuss this since the author will be watching there. *** This bug has been marked as a duplicate of 100206 ***