Have you ever tried to scroll down a page faster than your browser/computer can handle? You quickly flick the mouse wheel a few times, and when you are done the browser is still trying to catch up. It moves a few lines at a time until it eventually gets where you want it.
InstaScroll solves this problem. If the browser has fallen behind, InstaScroll causes it to instantly move to the accumulated scroll position all at once, instead of performing multiple moves of a few lines at a time.