E.g. In gnu make, the program keeps a dependency tree for what files depend on what other files. So if one file is changed make doesn't have to 'make world'.
In Sheets does it keep a topological sort of cell dependencies so to do the minimum recalculation?
Example:
Will sheets spend time doing the lower part of the orange box?
If the data in the yellow box changes, will ONLY those cells dependent on the yellow box be recalculated?