A plausible explanation is that it's doing some kind of preprocessing of the file into a data structure that allows for faster or more responsive scrolling and

A plausible explanation is that it's doing some kind of preprocessing of the file into a data structure that allows for faster or more responsive scrolling and editing of huge files. For example a rope or similar: https://en.wikipedia.org/wiki/Rope_(data_structure) https://en.wikipedia.org/wiki/Rope_(data_structure)