Is XP generally faster than Vista or 7? Yes. But to me, that's like saying that your car would be faster if you took out the comfortable seats, the air conditioning, the power steering, the sound system, etc. The car would be lighter and thus faster, but it would also be much less pleasant to drive. The same applies to XP - yes, it's faster than Vista/7, but all the things which are missing to enable it to be faster make for a much worse user experience. Most of us don't race our cars, so we'd take a slightly slower journey over a less comfortable one. In the same way, when I'm on my PC all day, I'd take a more pleasant experience over one that's a bit faster.
I don't know where people get this idea from that new OSes should use fewer resources than old ones. New OSes can use more resources because there are always more resources available due to advances in hardware.