Extremely poor bulk file copy speeds on AFP shares in Yosemite
Here's how I would attack the issue. It's not an answer, but hopefully we can crowd source ideas until you can report success or at least a way to measure things.


Set up a test case client with no third party apps running at log in. Reboot that client and mount the network share. Run sudo sysdiagnose Finder before you start a copy.
Start a tcp trace on the network adapter you will copy the file. If you aren't connecting over en0 - use System Information to see the BSD name of the network connection. 
Once the trace is started, start the copy of the file in question.
After 3 minutes (or less if the transfer is done sooner), press Control+C to end the capture
Run a second sudo sysdiagnose Finder after the network capture


With this slow of a transfer speed, something seriously amiss is happening in the network stack, but without looking at the client logs, it's going to be hard to know for sure what's halting the operation. You might also run a sysdiagnose on the server side once at about the same time as you do on the client side to eliminate a slow server as the issue. It seems you have plenty of horsepower for the storage to move rapidly, but getting server side logs will help too:

sudo sysdiagnose
sudo /Applications/Server.app/Contents/ServerRoot/usr/sbin/serverdiagnose


The trace is:

sudo tcpdump -i en0 -s 0 -B 524288 -w ~/Desktop/AFPslow.pcap