It's worse then you think for Areo Glass.
Areo Glass works like Linux (with it enabled) and OS X desktops: It uses compositing technics.
What it does is that it renders all the windows in a off screen buffer, converts the image to textures and then maps that textures on 3d-rendered 2d primatives.
Since all that is being done in video cards then it's going to be fairly fast.
But the sucky part is that your not going to have ANY acceleration to when your windows being initially rendered in the off screen buffer. At least for 'legacy' Win32 apps.
There is going to be no GDI/GDI+ acceleration in Areo Glass due to the composition stuff. Avalon-based apps will be very fast, they will have full acceleration due to the DCE engine... But since Avalon does exist, other then in developer previews and betas, there will be no avalon apps at the time of longhorns release and by far the majority of apps will be win32/GDI style stuff for a couple years at least.
So if you have a mega card with lots of memory the moving the windows will be fast, the special effects stuff will be nice, but when your working in a paticular window it will be much slower then it is in Windows XP. For the most part you probably won't notice it.
The windows won't have to be redrawn as you move them around and your CPU's will be much more powerfull so it's less lilkely that you'd notice the performance hit when operating in a paticular window.
The nice thing though is that all this gets put on hold when your doing full-screen 3d apps like games, so the performance for games will be OK... although you'd have serious lag as you exit the game and the video memory has to be refreshed with the 512meg or so or texture memory that will needed to be uploaded to get everything working again.
I expect most people will happily go without Full-fledged AeroGlass for a couple more years after they release longhorn, which should give laptops time ot catch up.