For speed, the newer virtualization tools KVM, Xen, and Virtuozzo are presently substantially ahead of the present incarnation of VMWare. KVM requires the new "hardware virtualization" CPU's from Intel and AMD which must be mentioned here, since they represent a major industry recognition of the value of virtualization.
This article seems to be giving people the impression that performance of VMWare Server is indicative of virtualization tools in general, and that all virtualization tools slow down hosted virtual machines dramatically. This is simply false. I know hosting providers running 50 virtual servers on a single dual CPU box with thousands and thousands of users, which would simply not work if all virtualization tools had a 43% hit per instance.
Another key matter here is that the author fails to mention (or realize?) VMWare Server is crippleware. VMWare states explicitly not to use VMWare Server for anything other than testing because it does not have the performance or feature set of their full blown ESX and GSX servers. Also, while VMWare may be the oldest and arguably most mature virtualization suite, it is certainly not the fastest.