Just wanted to report the final state:
Complete new OSMC from scratch and this time NFS as network intead of SMB had a very small effect in time delay, but still 10 to 15 seconds without any reaction could happen. Also going back to a parent folder sometimes has huge delays.
But switching off the automatic tumbnail generation in media / pictures gives completly normal behavior. And to be honest a tumbnail generation which is lost next time I enter the folder makes not really much sense for me. Also the generation is not finished after the delay, I have to point on nearly every image in the folder untill every tumbnail is generated.
So I think, this could cleary be improved in a future version as well as a 4K output, but currently I can live with the workaround.