I have run into this problem on my Minix U9-H also. I am using WiFi because I have no Ethernet ports in the bedroom, but I really don't think that's the problem because iperf says I can get a solid 200 Mb/s throughput from my file server. However, my database has all the video library sources as SMB shares, and no matter what I do to the cache settings, they will not transfer faster than 30 Mb/s or so. This basically means 4K videos are not playable on the device,
Mounting an NFS share manually on the box seems to work much better even with no custom cache settings, but this is not a solution because of the shared database and the fact that the DSPlayer version of Kodi (which I use on another device) doesn't support playback from NFS shares. It's also still not quite perfect, although it might be given tweaked cache settings.
Really not sure how to solve this aside from having a long cable going upstairs, which is not going to fly. Even if I could figure out how to manually mount SMB shares on the Minix U9-H, I'd still have to use the same path on all the devices (since they share the same database) and "/storage/..." is not a valid path for my Windows boxes.
Spent all day playing with this and am just stumped. It feels like it just isn't possible to stream 4K over WiFi, despite the fact that it has a consistent network speed of over 200 Mb/s.
EDIT: What about UPnP? I wonder if that'd work, but right now I'm pretty burned out on this project. 