I think this issue began with the new version of mobile me mail. The scrolling rate is very low when scrolling the message list or the message itself. It's not that the scrolling is sluggish or non-performant, it's just that each scroll movement results in a tiny amount of scrolling. Issue is observable in shipping Safari 5.0 and Chrome 5.0.375.86 on Windows, and not observable in shipping Safari 5.0 on Mac (didn't try Chrome, could be true there as well).
rdar://problem/8137702