This whitepaper describes tips and methods to help you migrate your Microsoft Virtual Server 2005 virtual machine from IDE to SCSI bus. You can increase overall virtual machine performance in most scenarios up to 20% by using SCSI virtual hard disks instead of IDE.

https://www.microsoft.com/downloads/details.aspx?FamilyId=8D71F23D-0380-4C2F-81DF-6F96ABE05493&displaylang=en