Well, take that article with a grain of salt. It is a few months old, improved XP drivers could be a factor if the tests were run again today.
Also, with Win2k SP2 against XP gold; SP2 has greatly improved performance for 2k on a number of machines, so that test may not be the whole story. It is interesting, but I'd like to see some newer tests, maybe after SP1 or 2 before drawing too many conclusions?
Edit *
Re-reading the article I found that one of their major complaints as far as speed goes had to do with databases:
"Overall we are quite disappointed with Windows XP's ability to pull serious weight when compared to Windows 2000. We are not certain where the problem lies. Our follow-up testing indicates that the additional database and multimedia workloads are breaking the proverbial camel's back. Microsoft claims it's been unable to duplicate our results, but hasn't supplied us with a better explanation or identified a major flaw in our testing. Whatever the cause, until the problem behind Windows XP performance is resolved, we can't recommend Windows XP as a client for serious database crunching."
Take a look at this article on 8wire.com
Performance Issues Plague XP Mdac 2.7
"Attention all database admins: There's a rather nasty bug lurking beneath Windows XP's shiny new facade, and the Microsoft Data Access Components (MDAC) group is to blame. It appears that someone on the ADO development team inadvertently "broke" the support libraries used to conduct client/server database transactions under MDAC 2.7, the version that ships with Windows XP.
Any applications that use these libraries - specifically, the various system DLLs that begin with "msado" - have the unpleasant habit of chewing up an inordinate amount of CPU time. Basically, they become CPU hogs as the buggy ADO libraries they interface with gobble up processing bandwidth, forcing the application to compete with other CPU-bound tasks for an ever-shrinking pool of processor time."
This could explain some of Infoworlds findings, hopefully an MDAC update will narrow the margin greatly.
Just a thought.