Well speaking for myself, I had a 5th grade teacher that had me come live with her for 2 months one summer, while she used me to beta test a new reading course.
Started out pretty much middle of the pack for my age group, and in 2 months was reading at the advanced college level for both speed and comprehension. Top that with some 45 years of daily practice, mostly reading for enjoyment, and well, its not that terribly hard. I think it helps to have a large vocabulary.
Like anything else people do, practice can improve it, especially when coupled with good habits. Not that I read that fast all the time, but my wife does give me a hard time regularly about just sucking the words in, but not chewing them.
