When I hit "preview" in the Editor I get the "Loading Screen".
The bar goes to 100% once.
Then it starts again and the bar goes from 0 to 100 again. But the time it takes is always the same.
I figured out, during that, it doesnt really load anymore, it just shows this "animation" till its finished.
If ALT-TAB out of the game and go in again you instantly get into the mission.
This is quite annoying for testing/previewing missions.