Bit Tech has just published their look at Windows Vista SP1 Core Performance:
"Overall, there are the occasional epic performance improvements and some epic losses, but for the most part the performance is very consistent. Considering all the additional fixes – the compatibility improvements, the security fixes and other tweaks, we'd suggest Windows Vista SP1 is worth installing unless you do something where there's a big performance drop… a lot.
. . .
For the most part though, having used Vista SP1 RTM and the full release for the best part of a several weeks now, we can't say it'll sway more people into buying the OS – the improvements are subtle and virtually unnoticeable to the end user... both looking from core application performance and gaming performance perspectives. And one thing Vista SP1 definitely isn’t is another Windows XP SP2 – most will still consider Vista to be "bloatware" (it's been well documented as considerably slower) because the "performance improvements" are nowhere near what was needed to change people's minds.