When you build an Unraid VM and use virtualised drives that are VMDKs sitting on datastores, are pre-clear times faster than when you pre-clear an equivalent sized physical drive or about the same? I'm about to embark on a hardware upgrade with a move to ESXi and i'm curious how long pre-clearing a 2TB VMDK will take.

I'd expect it to be notably slower.  With ESXi you'll get much better performance if you pass through a controller (or 2) for the drives, and directly connect them to the virtual machine.

 

Thanks for tip, i'll look at doing it that way instead.