Help me understand a bizarre phenomenon related to Full Map Capture in the og game

I swear this question has been bugging me ever since I was a kid who started to play this game.

So most of you veterans might remember the famous "F12" shortcut in the original game which prompted a capture of a full map screenshot.

Everything's fine so far, right? You'd see a progress bar slowly filling up as the game processes the image...

HOWEVER, whenever you quickly wiggled the mouse/cursor or moved it from a side of the screen to another in fast succession, the progress bar would actually fill up WAY faster and the capture would be complete in less time.

If you waited still, suppose it would take 30s+ for it to finish. If you wiggled your mouse, though, it would be completed sometimes in less than 10s!

How in damnation can this happen? I mean, what's the technical explanation for something like this? I can't be the only player who tried to do this, am I?

I swear I'm not crazy. I did this procedure dozens of times. If no one else has heard of it, I might install my original copy of the game tomorrow and capture a video of it in action!