Dunno. Perhaps the RAID5 overhead on that operating system is much higher than I'd expected. Dunno.
Isn't the RAID work being done by the controller?
That article, from 2007, refers to VMware server, which could be referring to the hosted product that VMware used to offer,
I think it can still be downloaded, but VMware have not made any changes to it in over 2 years. One of the links from the comments does clearly refer to server.
From your first post is sounds like you are using the ESXi, which can also be used for free. However, it does not do any RAID work, it leaves that up to the hardware.