Win XP never was exeptionally bad for games, however like with vista, the core of the OS is drastically different from the last.
In that respect it does not run OLDER games or older software designed with another OS platform in mind at 100% efficiency, as is to be expected.
XP was usable as a work (it was rock stable and fast for graphics/publishing work) OS from the very first day it went retail. Hell i remember not turning my work computer off or rebooting it for just over 100 days after I had installed and set it up the first time. Without badly written 3rd party "tweek" or "utility" software XP just worked and still does.
Benchmarking this build of Vista, which btw is a few months old already with minor tweeks for public release and comparing the performance to another OS is still absolutely pointless and it is meaningless to build any sort of opinion on Vistas final performance; game or otherwise, on any number shown in this article, since as said, a good portion of the OS is running in Debug mode still, which in turn means its using alot more memory than it will be, and running drastically slower than it will in the end.
It is similar to comparing a half cooked meal to a fully prepared one and asking people to make a serious judgement call on which it would like to eat in the near future.
Going over Vista and pointing out what the author likes and dislikes about the beta is one thing, but trying to benchmark it at this stage is just plain futile.