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.
Except that if you buy a Mac, you *can* run XP natively full speed, with drivers, and bypass the Vista disaster entirely!