Problem solved - the only way out was to do an as clean as possible reinstall.
You will lose a lot of your setups but your music files and the library with thumbnails are intact. But I did make a backup of everything. And it makes one wonder why JRiver spreads out files, cache, temp etc all around the C: drive. Why not put it all in one folder.
So I did try config the Defender, not to bather JRiver related files and folders. This did not alter the problem. And I am not really surprised. I have never seen that defender would slow down database read or write operations that I do with large databases on the same PC. But I shall do some more test once JRiver now is playing along.
I finally did reinstall JRiver 24 64 after trying everything else. So now JRemote also works as it did before my troubles started. The skip from track to track is almost instant.
I have tried to remember what kind of configuration I did do when my problems started. I have a feeling that I did fiddle with the Tools/Options/Audio -Track Change and -Stop, Seek & Skip settings and View/Skin trying out different skins.
I shall continue testing and see can I corrupt JRiver again.