We all know, what is VMware Storage vMotion, but have you ever thought about the process which follows in the backend.
Once you initiate Storage vMotion, it would first try to use the Hardware Offload feature, as that is the fastest. If that fails, it will use Software Data movers (FS3DM or FSDM accordingly).
Wondering what are these!!!
A Data Mover is a component in VMware vSphere that is responsible for migrating/re-locating the Data. Here is more detail about Datamovers
We have three Datamovers

This is Generic/Legacy Datamover. During Storage vMotion, if a different block sized vmfs volume or a different array is selected as the destination, the hypervisor uses FSDM. This would reclaim the Zeroed Space.

This is specific to VMFS. During Storage vMotion, if the same block sized VMFS volume is selected as Destination Datastore, it will use FS3DM, provided VAAI is not enabled. If it is VAAI Enabled Array, it will use Hardware Datamover. VMFS has intelligence about the allocated blocks which it uses to copy the data. This is used only when the source and destination VMFS have the same block size otherwise, it falls back to FSDM. It is not applicable for NFS. This can also be called FS3DM without VAAI.

3.Hardware Datamover:
This requires ‘VAAI’ need to be enabled. Both NFS and VMFS supports this, provided it is enabled at the target. For NFS, if required, the VAAI plugin needs to be installed.

NFS doesn’t have the intelligence of allocated blocks. For thin disks also, it reads the entire disk, even if the blocks are not allocated.
So if the source is NFS then these redundant reads, add to the copy time. Unfortunately, this is the limitation with the NFS, as currently there is no way to get the block details.