So if your whole network is gigabit, I would have thought it should work
Using PMP I just tried playing back Avatar from my NAS (DS1815+) to my Desktop and it plays fine, and that’s direct playing at about 131 Mbps according to the plex dashboard.
Edit.
You could try using PMP instead and see if you have the same issue.
It’s not on the downloads page, but you can find it here
