Excel has an internal formula to determine which cells have to get updated
based on the cells that got changed. The formula execution time will be
different depending on which cells have been modified.

I have a pretty complicated worksheet (23MB). As such, I keep the
calculation mode set to Manual. When I hit the F9 key to calculate, it can
take either 4 seconds or 15 minutes. The latter finally calculates properly
but EVER SO SLOWLY. Can someone explain why it seems to get hung up
sometimes when other times it flies through the math?

Thanks