That...is a fascinating question. Usually when dealing with verbal thought, it turns into voices in my head, unlike the OP. That might be why while I can read pretty fast, I still can only do so about carnival barker or auctioneer speed, rather than faster than speech. I could be automatically limited in my rate of comprehension due to how it's being processed, still as a 'voice', rather than visuals that map directly to concepts. I would -love- to see survey results on that, whether people had tried to speed read, if, having tried, they succeeded, and whether they tend to/ever have an 'internal voice' in their minds when thinking