Since both of my posts have been named on your Post, I’ll put my two cents in.
Lets start with the fact that network access and network speed has nothing to do with WD meaning that WD tech support with their scripted 40sec reboot will not solve your problem at all. To be sure, turn off cloud access as well as media servers like DLNA, iTunes Server and so forth so that your PR4100 isn’t bogged down with scans.
Since Network Troubleshooting has nothing to do with WD, you can forget about screaming about how bad WD tech support and their products are, after you RMA your WD device for the fourth time.
The second thing we need to clear up is making sure that when we say 80Mbps (Megabits per second) we are really saying 10MBPS (MegaBytes per second) which is reasonable for Wifi N as this is the practical max.
As a side note and when you have finally found your problem, you can get 40MBPS with Wifi AC which entails getting an Wifi AC router of which you can bridge it to your existing router by plugging an ethernet cable from your router to your new router using one of the local ethernet ports and not your WAN port. Google this on how to bridge two routers; remember to turn off DHCP on one of the routers as well as turning off Wifi on your old router.
The last thing is that even though you may have a Gigabit ethernet, apparently even today if you have any 100Mbps devices like printers, home security, lutron hubs (home kit) on the network the whole network could slow down to 100Mbps. If you do have 100Mbps devices you will discover this while troubleshooting your network.
Remove all devices from your network and only have your PC/Mac and your PR4100, both connect by gigabit ethernet on the network. Map the NAS and copy a 1GB file to or from the NAS. It should take about 10 seconds which is about 80MB/s to 100MB/s. If you are getting 10MB/s then your gigabit router has switched itself to 100Mb ethernet, or your ethernet cable is Cat 5 which may not support Gigabit speeds.
Check if your NAS has a Green led at the ethernet port. If yellow that is indicative of a 100Mb line.
If you still cannot get a 10 second copy of a 1GB file, you might need a switch between your router and your PC/Mac.
Regardless you should have a switch between your router and your home network anyways which makes it easier to swap out the router when you need to do so by just yanking one cable instead of a plethora of cables that goes everywhere.
Once you have determined that everything is working between your PR4100 and your PC/Mac start plugging in your other devices.
One last fact. If you have both wifi and ethernet connected, sometimes your connection goes through wifi instead of the gigabit ethernet. So disable Wifi if you are using ethernet.
So that is it, I think.
Let us know how your case is handled with WD. I could be wrong about the network having nothing to do with WD with WD tech support magically giving you an answer that none of us here in Customer supporting Customer land realizes or expected.
Also if you RMA your device, let us know how that turned out for you.