My video:
https://www.youtube.com/watch?v=bQNno57eYYg
First shows that it works fine on laptop screen and mirrored and extended modes.
Then shows how it works fine with only secondary screen running but only when laptop is still open.
Then shows how the error behaves. Everything lags heavily. Windows, menus, mouse pointer, video. The computer becomes useless and no work can be done. But strangely enough some graphics is not affected by any lag...