lol, did you miss his *sarcasm*? In technical aspects it is a downgrade, yes. But for many users without 'higher-end' PCs Vista is simply to 'advanced'(being nice with terms here) for the PC, so of course it will run rather slow. XP however is well suited for these PCs and so essentially here, you change to a different OS and get better performance, thus making it an UPgrade
As Spartan Erik said though, the Areo theme can be customized. The 'Vista Basic' theme uses no transpareny or anything so it would probably give you the best performance as far as the GUI slowing down your PC. Vista is simply 'heavier' and doesn't consider older or mid to low end PCs so if your specs aren't quite up there, Vista just won't be a good experience for you. Even running 1GB with a 3-4GHz processor and all won't do well on Vista(well, maybe it can do decently with Basic).
I'll leave with the advice you would hear from an MS tech support agent:
Have you thought about upgrading your PC? I think you need better hardware. Buy another one of our products to improve performance. Please hold. It seems your version is not supported.