Tom's Hardware published an interesting article comparing Windows Vista and Windows XP. They tested how fast certain applications and games run under Windows Vista compared to how fast they run in Windows XP. Not surprisingly, they conclude that, "Windows Vista clearly is not a great new performer when it comes to executing single applications at maximum speed...we clearly advise against replacing Windows XP with Windows Vista if you need to run professional graphics applications."
However, this doesn't mean that we should all avoid Windows Vista and stay with Windows XP forever. Remember that Windows Vista was re-written from the ground up. There are many new components and features built in to Vista that may make the upgrade worth it. "Vista is the better Windows, because it behaves better, because it looks better and because it feels better. But it cannot perform better than Windows XP." I'm sure that if I still ran Windows 95 on my current computer, it would run blazingly fast, but would I really want to do that? (Source: Tom's Hardware)