James Bannan of APC Mag posted an in-depth analysis of Windows Vista Service Pack 1

First up, I can say that there is a very noticeable performance increase. It is obvious that since Microsoft released Vista to manufacturing, it has been optimising the code ever since. (I suspect this revelation will fuel the fires of the people who say Vista was released before it was ready for prime-time.) There’s far less hard drive thrashing and in general the system seems much smoother and more responsive.

Vista SP1 analysed in-depth