I imagine, if you can comprehend at a very fast speed, it gets easier to keep the line in your head. As you can store and revall the characters from the very short term memory. I don't know if this phenomenon exists, but if I,'ve heard the entire line in for example 0.5 seconds, I think I'll be able to construct a mental image of it and code.

Another point is that I imagine it takes your complete focus to listen and comprehend single characters at such speeds, so you will be super-focused on the task when you're writing code.

We, as the programmers with sight, can read code without getting anything out of it, if we're not focused.