There are at least three places in which there should be a cessation of CPU loading from gameplay: load/save file, options menu, and pause in the pause/go selection. I can understand some CPU loading while editing the environment while paused, however shouldn't that limited to the requirements editing?
At the very least, there should be a mode where all CPU loading is ceased. This is life in the multitasking environment.
No need, CPU is continually loaded.