I think there are physical and mental barriers at a genetic level that, while maybe not making it impossible, introduce severe diminishing returns past a certain speed. For example, I literally can't read fast enough to exhaust my physical speed. Others might have the opposite problem or a mix of both.
I think the average upper ceiling is around 100wpm, but this probably differs from layout to layout.
But yeah, it's definitely muscle memory that allows you to touch type and the only way to develop this is repetition while not looking/cheating. Better off starting slow, but CORRECT, and just practice, practice, practice.