Try printing an incrementing integer on each line. Then if you have any lines in a row with the same number, you'll have something visually distinct to watch as they scroll rapid.
Regarding your question about how the serial monitor updates its display, the visual results vary depending on your operating system. On each system, results can vary depending on what other programs are active on your machine and how much CPU time and memory they are using. Some Linux systems even limit the number of refreshes per second depending on whether the mouse is located within the window. There simply is not 1 single answer to this question, and at least on this thread I can't even see if you're using Macintosh, Linux or Windows.
An incrementing number on each line should make this clear to see.