Ok, while I would be surpized if there was reduction in performance of the Kodi NFS implementation but maybe together with the unstable network it gives the issue.
May I suggest you try autofs to see if it solves the issue. If that is the case we may can revisit Kodi NFS again.
Just as an FYI, I used Kodi’s NFS on a Vero 4K+ on Kodi 18 (which was fine), but when I moved to Kodi 19 during testing, that was unusable for me with 4K movies even with 900mb+ throughput according to iperf3. I ended up changing everything over to using NFS via mounts and all my issues went away. I never took the time to look through the Kodi change logs to see what might have happened, but it is possible that the native NFS in Kodi 19 needs better/more stable/more bandwidth to achieve the same result.
Ok, managed to set up the network share with autofs and now it is playing flawlessly.
See screenshot, first peak is the buffering, the flat low bitrate piece is a black screen with the static movie title, and the next big peak is skipping 10min ahead. During playback you see this movie has a 100Mbit/s rate, and while skipping forward/backward as much as it can handle.
So to me it is clear the new kodi 19 internal nfs access is limited at approx 50Mbit/s, which is not going to cut it for these high-quality streams. And I am not the only one bumping into this it seems.
So while I got it up and running now (just need to rescan my library), I am sure more people will bump into this replacing half their network before realising it is just a limit of the kodi 19 nfs implementation, so definitely worth a fix or a warning somewhere.
It’s good that you’ve back up and running, but you haven’t found out why your network is performing so badly. Receive speeds of 500 Mbps and send speeds of 650 Mbps are too low. (Iperf3 measures the low-levell network speed, irrespective of what data transport layer you’re using on top.) As @fzinken said, the network speed should be around 920-930 Mbps both ways.
Clearly something is still wrong on the network side and by moving to a kernel-based mount you have managed to mitigate the problem, but not fix it. It might, for example, bea limitation of the way Kodi 19 has been configured to use libnfs that has revealed the underlying network problem. I came across this thread, admittedly five years old, that discusses such matters.