Many thanks to all for your answers! Now this is cleared up for me...
Hello, yesterday I bought and installed Affinity Photo and I'm quite happy with it... However, now I looked into the preferences and there I read (in German, of course ;-)): "No compatible GPU" (I attached a screenshot) That would be a surprise for me, because the Radeon Pro 580 is the fastest in the iMac-lineup, with the exception of the iMac Pro, of course. Is that normal? Will it be fixed with future updates? Greetings from Hamburg, Sören P.S.: macOS High Sierra, 10.13.3