Even if they calculated every single frame this really shouldn't impact performance unless the optimization is terrible. Maybe it's just a language thing, but I could perform the square root of 7727 a million times in just 4635428 nanoseconds (~1/216 of a second) with a Java loop. That's far more than needed and they should indeed only be updating it when there's an actual change.
Although speaking of optimization I wonder why there's a stutter when saving. Not that I mind considering it makes identifying when the game saves easier.