From: Tomshardware.com (major benchmarking website)
There are some programs that showed deeply disappointing performance. Unreal Tournament 2004 and the professional graphics benchmarking suite SPECviewperf 9.03 suffered heavily from the lack of support for the OpenGL graphics library under Windows Vista. This is something we expected, and we clearly advise against replacing Windows XP with Windows Vista if you need to run professional graphics applications.
We are disappointed that CPU-intensive applications such as video transcoding with XviD (DVD to XviD MPEG4) or the MainConcept H.264 Encoder performed 18% to nearly 24% slower in our standard benchmark scenarios. Both benchmarks finished much quicker under Windows XP. There aren't newer versions available, and we don't see immediate solutions to this issue.
Windows XP vs. Vista: The Benchmark Rundown | Tom's Hardware
I have to say, that Microsofts new operating system seems (at this moment) to be a disapointment in performance. Its possible in 6 months or so, it might be as quick as Windows XP with new software patches. BUT currently there are things that still do not work on Vista. And performance is worse.
If this were a pizza, it would look great, but taste more frozen than fresh.