As with many skills, natural aptitude and training probably both have effects on functional ability. Sounds like your DS has made a very strong start early.

Reading aloud would definitely slow a speed reader down immensely. I would think that mechanical vocalisation of the sounds would be a slow and rate limiting step requiring a lot of coordinated neural & neuromuscular synaptic activity. Even silent subvocalisation slows reading speed compared to simply processing visual information.