I can read pretty quickly (I've never really measured, but it takes me about a minute to read a page, which I guess is around 500 words). Depending on the text, though, there's a lot of slowing down or re-reading. I read Harry Potter pretty quickly, but "Thinking: Fast and Slow" is nowhere near at the same pace.
Yes, a lot of times speeds keep on varying. It depends a lot on the type of book that you're reading. For the rest, it's inherent abilities that assist.
