Powered Off Storage Migration and Performance
If you are moving a powered off VM from one host to another, that cannot see the same storage, the copying of the data is send over the network.
In version 7 and older this is done with the NFC protocol, but that is slow, so with vSphere 8 VMware has introduces a new protocol for doing this called Unified Data Transport (UDT). UDT combines the best of the NFC and vSphere vMotion protocols.
To use this new feature, you must enable the “Provisioning” service on the desired VMkernel.
VMware has a demo on YouTube of the different speed for this here.
Note: I also just found this blog about this, with some more information on the this here.