XP is an older system and has had time for most stuff to reach optimum performance. Vista has not been out for very long and has not reached peak performance. Some parts are still slow, and combined with that is the fact that Vista itself is more demanding on system resources. So of course games are going to run faster on XP than Vista.
At least until DX10 gets optimized. My understanding is that DX10 moves most of the graphics stuff off to the GPU instead of having the CPU handle the fancy visuals as well as everything else. It may also be that you have to have a DX10 graphics card for the offloading to work.
I have both Vista and XP on my computer, Vista is visually more pleasing, as well as more secure (debatable), has a ton more performance monitoring features, and has DX10 (although there aren't many games out yet that use it. And why does Flight Simulator X use DX9 when it is supposed to showcase the power of DX10?). But I use XP for gaming because it runs faster.
At least until DX10 gets optimized. My understanding is that DX10 moves most of the graphics stuff off to the GPU instead of having the CPU handle the fancy visuals as well as everything else. It may also be that you have to have a DX10 graphics card for the offloading to work.
I have both Vista and XP on my computer, Vista is visually more pleasing, as well as more secure (debatable), has a ton more performance monitoring features, and has DX10 (although there aren't many games out yet that use it. And why does Flight Simulator X use DX9 when it is supposed to showcase the power of DX10?). But I use XP for gaming because it runs faster.