@GRIEVER I have quite the same problem. 1 of my deco units is being throttled when connected to ethernet. It happens randomly and gets fixed when I unplugg and replug the ethernet cable. It does not happen when I dont use the ethernet cable. 

Only 1 slave is connected to the main router via ethernet. The other slave ks via wifi to the main deco. It does not have issues woth the speed. 

Now, I dont want to let go of the ethernet backhaul since I have my pc connected to that slave unit and I use it for gaming. 

Any advice? 

Thanks!