When copying files between two VMs (esxi, latest version) on two different hosts, file copies move slowly - about 20mb/s. Copying to physical servers from the VMs also move at about the same speed, ...

I'm going to assume this is from the perspective of KVM. If its for Xen, some of the below may or may not apply. Generally, LUN's are per VM, so any export limits on your storage and LUN count ...