Except that by doing so, you just lost a good 20-25% of the speed over native XP, since Vista adds that much overhead.
Granted, but compare that modern CPU with many GB of today's cheap RAM (that only runs Vista native), to an older CPU with less memory (that will run XP native), and the 20%-25% disappears. A factor of 2 would hurt, but it's nowhere near that bad.
Yes, if you COULD run XP native, you'd get another 20-25%. But you can't, so don't sweat what you can't have.
CPU power is cheap and getting cheaper. RAM is dirt cheap and getting cheaper. Don't fret about a small loss -- it's invisible in the overall advance of the technology.