I know now, how I have to speed up my very slow Discover option.
Probably it is my network configuration that is the real culprit for the long delay. I need a special setup because Roon Server doesn’t play nice with my Livebox Play from Orange (a French internet provider).
Accidentally is have discovered, that going to History solves the problem.
Yes, I know this sounds weird, but I have tested it several times (for example after a power failure). It always works.