In Windows 8, when you minimize a game and switch to it back again, then the game will be displayed very fast. It looks like that the screen was ready even before you told your PC to show the game. When you do the same in Windows 7, then it will take a second till you see the game. This time it looks like Direct X was suspended and the Video card needs time to prepare the screen.
So the question is, does Windows 8 render the game in the background and does Windows 7 begin to render when it is going to be displayed? Or is this because a change in the Windows API?
Do you use the same hardware and DirectX run time library for your test? I don't think that's a OS related performance issue, since most of DirectX program/game will stop rendering when they were minimized in order to release CPU time to other programs