I have always had internal narration, and prior to my TBI, I could read about 100 pages an hour of technical information with better than 95% retention. Reading fiction was faster. It wasn't uncommon for me to blow through six or seven 400-500 page novels in an average weekend while still having plenty of time to do other stuff. So I don't think that listening to the "audiobook in your head" is necessarily the reason for a slower reading speed.