From what I understood when Dr. Vincent was talking, even though the website changes the duration between dits and dahs based on WPM, it becomes very hard to understand when it is too slow. Thus, he recommended we keep the intraspacing (spacing within a single character, such as the dits and dahs) constant. This means changing the space between characters and words at a consistent ratio while keeping the spacing within the dits and dahs unchanged. I hope this makes sense.