Bad streaming performance over LAN with D-Link 850L supplied by Time.

I was using a Xiaomi Mi Router 3 when I was on Time 100Mbps plan, but decided to give the 850 a try after being upgraded to 500Mbps.

I stream through a Cat 6 LAN cable direct to my Android TV box which runs various APKs like Swift Streamz, Mobdro etc.

With the Mi 3 streaming was pretty smooth, but with the 850 it's terrible - lots of buffering on all apps. It's weird because when I test the speed with the Ookla Android app (not in a browser) I get over 500Mbps down and about 350MBps up, which is waaayyyy more than I would need to stream. These streams are just SD or 720P at the most, and the Mi 3 (limited to 100Mbps WAN-LAN) copes with them just fine.

What could be the problem? I'm not downloading anything else when I stream. Both radios are on but not in use. I have the latest FW 2.21. QoS is off. Puzzling!