A guy at the YouGamers forums did some benching to check the performance difference between Windows Vista SP1 and Windows XP SP2. You can check out the results over here.
In most of the application tests Windows Vista beats XP but in 3DMark and Crysis it's a bit slower:
It's interesting to see that Vista's performance seems to have progressed since it was released. The general usage tests are either faster or equal to XP. It should be noted that the program load times may be influenced slightly by the fact that the OSs reside on different drives (Samsung/WD). However, Vista is on the slower drive (the WD), so it only makes the results even more impressive. Talking about the program load times, it's obvious that Vista's SuperFetch feature works wonderfully. Even though I've only launched Photoshop and OpenOffice a few times, Vista has already picked this up and loads them into mem right after boot up. Crysis was almost certainly not cached into RAM during the test, but still loaded faster than in XP.
The horrendous file copy performance has been fixed. Performance is definitely higher than in XP. Also gone is the sometimes irritatingly long file delete times. File deletion seems instantaneous now, just like it's always been in XP.
Now, we've come to the less stellar part of Vista's performance: Gaming. 3DMark06 did perform okay, with numbers within 3-4% of XP. Although a performance decrease is never welcome, this is tolerable. Moving on to Crysis, however, things look a little more bleak. Performance is down 5-8% when looking at DX9 32/64-bit benches and DX10 is even worse off. The CPU test is also curious with a WinXP performance lead of 15%! This was definitely unexpected and something that would be interesting to research further.
The guy also did some tests to see of Windows Vista's SuperFetch feature makes a big difference. Apparently it does have a huge impact on program load times, the tests results show some applications boot up to 4x as fast with SuperFetch activated.