I was scratching my head recently about why moving Netapp volumes on my filer was going so slowly and just ran across the reason.
Here is the Netapp TR on DataMotion which explains in further detail what DataMotion is and does: NetApp DataMotion for Volumes
The reason why it was going slower than it should was the fact that the "vol move" command is using snapmirror to actually move the data. In our case I had enabled options.replication.throttle so as to make sure to not beat up our WAN with snapmirror traffic. This throttles all snapmirror traffic on a given filer so my local volume move was limited to the limit I had set for moving data across the WAN which was far from ideal. So if you're hitting the throttle setting either bump up that limit or temporarily turn throttling off (options.throttle.enable off) if you'd like to move that volume faster.