Vista is already slower than XP.
That's news to me....on my XP/Vista dual boot rig, Vista is actually faster than XP, both at startup and running apps/processes, etc. (AMD Athlonx2 4600+ 2.6ghz (dual core), 4gigs DDR400 RAM)

Think it has much to do with the PC its on....naturally Vista will struggle on an older/lower end machine with insufficient RAM and CPU, but should be quite fast and stable on more modern/high-end machines....
For example, my nephew bought a new PC with Vista Home Premium pre-loaded and was complaining it ran "like crap" and kept freezing up....I took one look and saw he only had 512mb of RAM, and certainly not enough to run Vista smoothly. I upped it to 1gig to start with, but now he's running it on 2gigs of DDR400 RAM, an AMD Athlonx2 4400+ 2.2ghz (dual core), and is now crowing about how much better his PC is compared to his mother's and sister's, who both are still on XP Home.....
It's sort of akin to trying to run a high powered/highly tuned NASCAR on mower fuel....it will run, sorta, but nowhere near its optimum performance.