It's a power-saving feature of the driver. D3D doesn't do any power saving. The problem is that the core speed doesn't come back up, so it's a driver bug.
I had a similar problem once: whenever I'd wake the PC from sleep, the GPU core speed was stuck at the speed it was when I put it to sleep. Maybe you're haveing the same problem - then it has nothing to do with the fact that you minimize your window, nor with D3D.