Hi Ryuu,
Sent a problematic scene to you yesterday. Working on another bathroom scene with no environment light and the same thing is happening. The really weird thing is that if I render normally then it uses 5% cpu. If I stick a render region on part of the view and then the cpu jumps up to 70%-100% usage. So I can render a scene but have to move the region around.
Also the same thing applies in interactive, 5 % normal, zoom in and it jumps up to 60-70%, then drops back down when zoomed out.
Updated: even stranger, region render at the top of the image works 100%, bottom of the screen 3%, start moving the region up the screen and watching the cpu climb back up