I assume, that the rotations can be done within one frame - thus, the gauges are animated with full 60 fps. As soon as there are additional areas updated, the entire screen area cannot be redrawn within one frame. As a result, the effective fps goes down, and the amount of drawing operations per second are down. As a result, the CPU load decreases, because there are less drawing operations to be prepared and the waiting time increases.
Does this answer your question?