(Technically it doesn't wait for you to stop scrolling, but the way they've designed it it will attempt to render everything you scroll past and then, even if you scroll past vast sections of list you don't care about and never see that list item - everything gets rendered in order. So when you stop scrolling, it's still busy trying to render everything you've scrolled past - then getting ridding of them and rendering the next bits of data in the list until it finally reaches where you've stopped - largely wasteful but there are reasons why they don't do otherwise. On a faster processor it's likely it'll catch up with you whilst you're scrolling, but I hold out no hope for WinRT tablets being able to keep up, nor lower powered Intel Atom tablets. It doesn't help that whilst scrolling, the rendering part of it becomes a lower priority process, because keeping the scrolling smooth is more important from a usability point of view so more CPU power is spent keeping the scrolling animations smooth.)
They do need to do it for RAM and disk access reasons, but I really don't see why they forced themselves to use HTML & JS for all internal Microsoft applications, when there are times where XAML is clearly a better and more performant fit.
The performance is roughly the same on my antiquated laptop (T7500 with an Nvidia 8600M GT) as it is on my home machine (X3 435, Nvidia GTX 560Ti). Neither are top of the line machines, but they're also solid performers. Both fail to load content as fast as the mouse scrolls; it's really pretty sad.
In its current state, Windows 8 feels like it's about 90% there. I like a lot of the new features, and overall performance is pretty nice. You still bump into glaring failures though, on a daily basis; from the App store not being able to count updates (the Live Tile and text at the top corner say 7, the app only lists 5), to scrolling content that disappears, to the Mail app that always announces that it has failed to access my IMAP account, even though it still syncs messages... It's like knowing the world's fastest 100 m dasher who falls one out of every ten sprints and doesn't know how to tie his shoelaces.