GUI slowdown when using second GPU for preview rendering.

Hello there,

I have a machine here with two GPUs, one AMD and one nVidia. I have my monitor connected to the AMD card, so I suspect Windows (I run 8.1) runs on that card (it also says so in the Screen Resolution window). When I run Blender and switch Cycles to CUDA on the nVidia card, I have major slowdowns when using the Blender GUI while the nVidia card is computing a preview render in the viewport. Its almost unusable until the preview is done. This beats the whole purpose of using a second dedicated render GPU.
All I can think of is Blender somehow runs the GUI on the nVidia card also, how can I check and make sure Blender itself runs on the AMD card?

Please help me out here, I really want snappy and fast preview rendering…