I've tried the first public test version of Win 8 (booted from network, so I/O is not comparable to a local installation) on my T43 thinkpad laptop, and it definitely runs slower than Windows Vista which is the local installed OS.
I'm convinced because the graphics card, a Radeon mobility X300 w/64MB RAM, is too slow for the graphic effects in Win8. Windows Vista's hw-accelerated Aero interface is much lighter on the GPU. With Windows 8, every time there's an effect, the screen updates in a choppy manner. This is mostly in the supplied Metro apps, as they seem to use lots of graphic effects.