I can't think of a way to do this automatically

Ie in a game, each time the game loops for 1 frame, there are typically dozens of For loops to handle the different elements. Nothing in the code tells the compiler where you want to Sleep.
In some programs, or in parts of some programs (Ie. when the program is loading), you will want to use 100% of the CPU. In other places you may want to use 50-80% of the CPU for an action game, or 5-15% of the CPU for a utility. And in other programs, you may want to keep a certain frame rate, which will use 90% of the CPU on a P3, or 25% of the CPU on a P4.
There is no one solution for all these programs, except to put one or two Sleep lines in your programs, so the programmer has control over this.
With the improved speed in FB and QB64, your QB games will run too quickly, so you will need to slow the program down, anyway.