I find this a very interesting question. I figured out how to read fast when I was 7 years old. It's a bit hard to try to remember exactly how I learned it because it was largely intuitive, as I guess most things are at that age. For the most part, I would characterize it as impatience -- just in general, being a kind of speed demon when it came to all kinds of mental tasks, math and reading included.

I used to be very interested in reading and writing, and although I wouldn't quite say I read all the time, I still read a fair bit. We used to have tests at school where we had to read 200 words a minute out loud, and I used to clock in at 35 seconds, and I definitely used to read faster in my mind, so I have a fairly accurate idea of my reading speed.

A question: Is there a step between reading out loud and reading in your head? From what I remember, I could always read in my head, but it's possible that there was an intermediate step, maybe before I was 4 years old or so. Regardless, I remember that I used to subvocalize initially, and I know that this changed eventually by the time I was reading novels. I can't remember how exactly this took place, and I don't think it was a conscious process. I just wanted to go faster, and somehow I did.

Another interesting data point I have now is that of a friend, a guy who said he had a poor reading speed, in his early 20s. He used to read at 250-400 wpm, and he got into reading fairly difficult books, about a hundred pages a day. A few months later, he told me his reading speed had gone up to about 800 wpm if he tried. Another friend who was quite intelligent told me his reading speed went up pretty significantly after reading a bunch of philosophy. Both the friends were fairly highly intelligent (I don't know their IQs but they're in STEM grad school).

Given all of this, I suppose it's a bit of a talent, and people who are naturally inclined can probably pick it up at different ages. I've read research which suggests that due to the number of saccades the eye can perform in a second, you can't really read above 500 wpm. What I strongly suspect is that fast readers pick up patterns in sentences, and therefore they basically skip over words while losing basically none of the content. After all, a lot of words in a given sentence are filler. This is what I suspect. It's very rare to see someone who can read at a speed of above 1000 wpm, and I think this is indicative of the fact that, after a certain point, the benefit you can get from pattern-matching and skipping over unnecessary filler content, and the speed at which you can do this recognition accurately, reaches some kind of limit for virtually everyone.

From what I've seen, I think language skills of various kinds really are a fairly accurate predictor of intellectual ability. I generally observe that the more intelligent the person, the quicker they can understand the nuances of a new culture and language, even if they may not agree with it. This might seem a bit of a tangent, but several of the people I know are academics for which English isn't really their first language. Some learned it as a second language, some only really picked it up properly in middle school. But they all read fairly fast and have good vocabularies. They are not as fast as native speakers in the same situation, but they often figure out how to read fairly fast, skim through research papers, etc. pretty well if a tad slower than fully native speakers.

I've been asked by people to teach them how to read fast, when they were adults at the time. And I've always wondered how I could answer that question, so I've thought about this question often. I wondered whether it's different learning to read fast as an adult vs a kid, and whether that would be relevant to the discussion, but from what I understand of critical periods, there really isn't a critical period for learning how to read script, because it simply is too recent in evolutionary terms for something of the sort to develop (so the anecdotes I've had where people did become fairly fast readers as adults should be relevant). On the other hand, understanding of language might bleed into reading speed because of intuitive knowledge of sentence construction etc. so it's not completely clear. Being good at anything which is evolutionarily novel tends to be a good indicator of intelligence, so it makes complete sense for reading speed to fall into that category.

I know this is a bit rambly and I didn't have a lot of time to format it. I wonder if your experiences agree with this.