I don't pretend to know anything about HP calc hacking (did a little TI BASIC for fun, that's not even remotely similar). But doesn't this come up in all kinds of daily situations, graphical or otherwise? You only recalculate and update the parts that may have changed. I assume this is similar to why Zelda on my original black and gray GameBoy as a 90s kid was arranged into scenes with few moving elements, so it didn't have to recalculate rendering for the entire screen during action.