Well it isn't technically "eaten up".
The way it is organized is that display updates are run in the main loop (low priority) every 1/2 second, and sensor updates happen in interrupts (and interrupt the main loop). If we get to the bottom of the main loop, where we sit and wait for the remainder of the 1/2 second, and we don't have any time left, then we would be at %100 indicated. So all the computations to display and interrupts are taking about 1/4 second if the cpu is at %50 and the rest is idle time, sort of
(interrupts still happening)