Fullscreen games are usually the ones requiring of more resources. Windowed games generally aren't expected to hammer the system. If they are in a window, it is not unreasonable for a user to expect them to multitask in a friendly way.
If I run a game in a window, it's because I want to keep an eye on another task. I'd be disappointed to see that task devoid of all CPU cycles.
If I run a game in a window, it's because I want to keep an eye on another task. I'd be disappointed to see that task devoid of all CPU cycles.