Fastest means to copy many drives to one drive
Doing the math, you want an interruptible tool so use rsync over Finder unless you really don't mind losing a day of transfer if finder chokes or a cable gets disconnected. 


Safest way to copy files to AirPort Disk from the command-line?


Barring the ability or inclination to command line - get a tool like Transmit or Kaleidoscope to sync the two folders. You'll lose far more time mounting drives than just connecting them and going. He limiting factor is probably not iops and instead drive throughput and not CPU or adapter speed.