Disk I/O is the biggest hit because the VM is technically just a bunch of files. The vmx file (VM config) gets loaded into memory. The rest then is access the vmdk (virtual disk) and memory/CPU usage. The fight for resources is where you'll see slowdowns. I'd go to an even faster option if possible (10,000 RPMs). And where possible, avoid shared SCSI (exception is ESX and the use of Fibre Channel/SAN).