I have found that the brain gets increasingly overloaded running visual cortex lookups beyond 12-14 wpm. It is very capable at speeds of 8-12 wpm. Thereafter the overhead is too much. Auditory system processes access memory differently. If you try to remember a visual scene, even from a few minutes ago, it is amazing how much detail you will miss, or even hallucinate. Methods like the Koch training soon bypass the visual limit and 20 wpm can soon be reached even as you add more and more letters. I find the auditory feed relaxing, meditative, almost musical. My hand may sometimes note the letters on the pad without my conscious engagement.