I was having a lot of buffering hardwired connected to LAN, I thought it might have been my VPN but with VPN off or on I was only able to get around 20Mbps with +/- 5Mbps. Currently I am able to stream without buffering now using the wifi adapter that was included. But again, with or without VPN I am only seeing about 50Mbps, what's up? I know my desktop is able to get 1/4 of my full bandwidth speed of 400Mbps. Obviously I don't expect that much but I thought I would see atleast as much hardwired as I am currently seeing with the wifi adapter.
On my Fire 4k I found out the network adaptor is 100mb max so over wifi I get 250mbs due to it be next to the router, 5ghz and AC.
I would guess your adaptor is the issue - maybe buy a cheap android compatible USB gigabit one?