Resource Pool Management

Hi, I've recently created some resource pools for VM's or VDI's to be exact.

There appears to be different ways to move VM's to resource pools from the Default location.
The drag and drop method (sluggish with 100+ to move)
The select multiple VMs option from the Host tab within vCenter and using the migration option to move them to a resource pool. Can be a bit of a fiddly process when needing to move 400+ as you can only move in batches of "Powered off" or "Powered on".

I also encountered issues with the later, as I "migrated" the VM's in batches of about 40 within a short period of time, i noticed that vCenter appeared to really be struggling to "migrate" the VM's to the resource pool. This actually caused a complete clog of the cluster and prevented normal DRS work from taking place.

I might be a bit naive or lack understanding, but does anyone understand why the "migration" was required, essentially these VM's with no affinity rules etc were just being added to a resource pool no form of migration should be required right?

Maybe there is a much better way to do this for larger clusters with hundreds of VM's potentially a more failsafe powershell command to move a large number?

I essentially had to wait in this instance for the "migrations" to fail noticing that they were erroring due to resource contention on the cluster, again I didn't think any form of host movement was required when adding to a resource pool. 

Look forward to a response to further/update my understanding here :)