Why will 'Recompute' result in faster execution time?

When I open my GH file my execution time is 3x that, of when I hit the ‘recompute’ button. Same for certain components that go from seconds to milliseconds after a recompute. Can someone explain the reasoning behind this? Or a more general question when do you use a recompute?
BEFORE - after the initial loading of gh file

AFTER forced recompute

As with most computer code, the first time a program is loaded into memory, it needs to be interpreted or compiled into assembler language, which takes time, and then the assembler language runs. the second time, doesn’t need to be interpreted or compiled, just run. This is a general feature of most language based computer architectures, and is what actually happens on the hardware level of most computers. Embedded real time machine controls on industrial equipment are an exception, they are coded directly in assembler language (machine language).

More specifically to GH, the first time through, a component needs to calculate everything; the second time through, it only calculates what has changed.

These are gross oversimplifications, but still.