Unity application suddenly slow


I’m new to Unity and developed a very simple application, nothing more then sphere which can be moved in a terrain.
I used “Camera and Play Area - Unity Learn” (splendid tutorial by the way) to start with and made a good start, being able to move my sphere without any issues, not even with mountains.

But all of a sudden my Unity is getting very slow. Testing the application in “GAME” mode or running the app after building it, in both the ball is literly crawling over the terrain.

Does anyone has ever experienced this before?

Starting a new project and copy all stuff in it did the trick