Extremely slow file transfer between networked Macs
I have a new Mac Pro, previous generation 2009 Mac Pro, and a 2012 Mac Mini all operating on an Airport Extreme (latest model, latest firmware) wireless network. The New Mac Pro and Mac Mini are running OS X El Capitan and the Older Mac Pro is running Yosemite. I am experiencing glacial file transfer speeds between the machines. I have even tried bypassing the wireless network altogether and connecting the machines directly to each other via ethernet, with wireless networking turned off, and the problem persists. I'm am trying to backup a large iPhoto library which is 290GB. When I try to copy the files to another machine, I get the "preparing to copy" message for about 5 minutes, then the files eventually begin to transfer, but extremely slowly (something like 1MB every 10 seconds.) Sometimes if I let the file transfer continue, it will speed up for a while, but it eventually slows down again. I know none of the disks are the problem since I've tried various combinations of machines and destination disks, and the problem is still there both in wireless and ethernet transfers. Strangely, I can download/ upload files form the internet at normal (fast) speeds; it's just computer to computer transfers that are lagging. I have searched dozens of posts but I have yet to find any solution to this issue. Any assistance would be greatly appreciated.
Mac Pro (Late 2013), OS X El Capitan (10.11.1)