Ash on my head.
Sometimes you just have to sleep over it. Here was the problem again not in the PC but before. 😉

During my performance tests, I had activated the scopes while running the UHDp30 preview and also saw the full 29.97fps frame rate. After MS updates and tidying up I did not think about these now constantly updated scopes and the area was also covered by the Task Manager window. Now I have turned it off and get at UHDp50 both the full frame rate and again a 33% CPU load.

This PC will now stay offline as much as possible.