Stuttering on second display

I've got a weird problem. I didn't change anything on my system (except maybe Windows Updates). I've got a Raedon 290 with 15.4 drivers, Windows 8.1 and I'm working with two screens. As soon as I start using my secondary display the 3D Perfomance of my primary screen drops about 20%. What's even worse, everything of the second display runs sluggish. Applications, videos, even webpages. When I'm dragging a window from one display to another, it's stuttering like hell and takes serveral seconds.

Does anyone have the same problem? Can you recommend any real good drivers für Windows 8.1?

Thanks in advance.

Stuttering on second display