I remember a cute trick Windows did (at least in the ~XP days) where if a window had focus, it got more CPU cycles. You could, for instance, create two identical console windows running a program that just does math in a loop, and if you give focus to one of them and let them churn, it will start to out-strip the other.
Of course, the underlying problem is that it's not always easy to know where the user's attention is (or if they have walked away, as you mention), but it was a simple "somewhat right" trick.
Of course, the underlying problem is that it's not always easy to know where the user's attention is (or if they have walked away, as you mention), but it was a simple "somewhat right" trick.