I have had this issue for quite some time and have decided to try and find a solution. Have scoured forum after forum with no luck.
My system: i9, 64Gb RAM, updated, Plasma, 1Tb HD, connected to several mounts via NFS to TrueNAS.
When interacting with the filesystem through Dolphin the filesystem becomes unresponsive. I can change windows, do stuff on the web but have to wait for the right mouse button to react on the filesystem.
Am puzzled where the problem can be. I sometimes believe it is NFS but then if I unmout all or use the local disk I still run into the same problem.
Any pointers greatly appreciated.
I think I would start with looking at logs through:
journalctl (using grep to find specific entries perhaps, or --since to view the log around the time of the lag).
No expert, just my 2 cents.
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Does this happen every time you use dolphin or occasionally? You may try alternatives like Gnome files or Crusader or Thunar to see if that results in the same behaviour.
Hello SemLraug,
Thanks for your suggestion it seems it is some combination of NFS server and client so I am looking in this area.
Well, that’s at least a starting point! 
Please give us a follow up if you do find a cause and perhaps even a remedy. Your findings may be relevant to others with the same or similar problems.
And if you get stuck you could post some relevant journal info here. There are people here who know a lot more than I do and may be able to help you.