First of all, thanks for your answer!
Well, I have another Level and there isn’t the problem. Maybe I really have set up something wrong in the first one…
Another thing I noticed in the first Level:
When I start the Game in Viewport (Quality Level is ES2), the counter for visible static meshes goes from around 30 down to 0 in less than 5 seconds and the visible dynamic primitives number increases!
I don’t know why. But the performance seems to be the same like in the other level.