Grey, if you hog the cpu in window mode you may find you have a very difficult time even getting the focus to switch to a different window because the GUI may not get any time to handle those events. Catch 22.
I think maybe it's a good thing to try and get your framework as smooth as possible and to deal with other threads interrupting, but I wouldn't lose too much sleep over it. How do you know that the user doesn't WANT there to be other threads running or other apps in the background. Maybe they're playing a game while doing rendering. If they are choosing to put software on their system that takes up CPU time, that's their business, and you can only do so much to prevent it.
I think maybe it's a good thing to try and get your framework as smooth as possible and to deal with other threads interrupting, but I wouldn't lose too much sleep over it. How do you know that the user doesn't WANT there to be other threads running or other apps in the background. Maybe they're playing a game while doing rendering. If they are choosing to put software on their system that takes up CPU time, that's their business, and you can only do so much to prevent it.