How big are the files that you’re copying? Are they journalled?
How many disks did you have in the old machine and how many in the new machine?
It may be that you had lots of smaller disks in the old machine which would spread the load of a large copy operation across the disks – your new machine may only have a couple of (larger) disks which means the machine is unable to spread the load so well. I’ve recently noticed the same thing on a new machine. You should be able to see how many disks you have and how bust they are using the WRKDSKSSTS command.