Kodi on my 2015 Nvidia Shield doesn’t stutter for me playing back 30GB+ 4k files on a 1Gb network from and ancient (2012) AMD Athlon TrueNAS box. It could be network related, but you can test this from another machine (laptop, desktop, etc) or by using local playback on the pi. I have cheap network hardware, and have never needed better. All this is to say Kodi mounting NFS shouldn’t need much bandwidth or high end gear. Perhaps the issue is on the playback side. Good luck!