I have a similar setup but my nfs server is not a mini pc.
You could try diagnosing if it's the network or if your mini pc is too busy (maybe out of disk i/o?).
If that's too hard, temporarly stop all other programs on the mini pc. Does it help?
Does streaming something from the internet to kodi work without freezes? If it's ok, then network is likely not the issue
Last but not least, does playing the same video from an USB stick work smoothly? If not, maybe the quality is too high or the video is x265