When moving, you may see better singular vol move performance by minimizing the number of simultaneous moves.
When working especially with SATA drives, the more moves, the slower.
I tend to tell customers to do no more than two moves per source or destination, especially if the src or dst is SATA only. When using Hybrid or SAS or SSD, we can usually go to 3 or 4. Generally though, stick to two.
Also, if clients are accessing the source volume, that will slow the process down.
If you really, really, REALLY want to move it as fast as you can,
Use "set diag" mode and there is a "-bypass-throttle" option you can use: USE AT YOUR OWN RISK!!!
Using this option basically says that "it is more important to move the data than to serve the data!"
Also, if you want, you can also use something host side like XCP. It's a multi-threaded copying engine that can go as fast as the hardware will allow and will go faster. We have used it with smaller tier or older filers with petabytes to move from Infinivols to Flexgroups.