Branched Path Tracing on GPU blocks computer


On the GPU the branced path tracing blocks the computer (i.e. everything reacts with extreme lag ~10 sec., even the blender interface). On the CPU it does not block the ui, it’s just slow. Also the normal path tracing does not block the computer, i can even watch movies while rendering. Is it a know problem?

Xubuntu 13.10
GTX Titan, Driver 319.60