PI4 with OSMC to read folder/files on Windows10 via NFS

If you’re not sure what the raw network speed is, then you should use iperf3 to measure it. Run iperf3 server on the Windows box, and the client on your OSMC device.

See here:

And you’ll make it much easier for people to help you if you post some debug logs.

Go into Settings/System/Logging and enable debug logging; reboot twice; play a video that is giving you trouble (for a minute or so); then go into Settings/My OSMC, find the Log Uploader (clipboard icon) and manually upload logs; then turn logging off again. When you upload the logs it will give you a URL; post that URL here and the OSMC guys can have a look at the state of your system, and what it’s doing while playing that video.

You might also try installing MediaInfo on your Windows box and posting the MediaInfo data for a problematic video, just to see if there’s anything odd about the file.