I have exactly the same problem. I learned to read extremely early (before the age of 2), and read Lord of the Rings for the first time when I was 7. I devoured everything I could get my hands on.
I never took a speed reading course or lesson, but I cannot recall ever having subvocalized. I find myself hitting 750 wpm on speed reading tests with decent comprehension. Not great, but decent.
To use a computing metaphor, I feel like I'm reading with two threads - one to take in all the words as quickly as possible, and a second one to pick the important ones out of the stream. The first thread is completely unconscious, to the point where if you ask me to find a specific word on a page of text, I can always pick it out within 2-3 seconds.
But I feel like a greyhound, itching to run at top speed. Reading fiction is a constant struggle to keep myself chained, forcing myself to imagine the scene playing out, when all I want to do is speed ahead to find out what happens next. It's like an internal war, and it gets worse at the climatic moments.
I'm in the exact same place. I devoured fiction (especially science fiction and fantasy) in my youth (ages 7-17, including 6 full LOTR readings), but the appeal has been destroyed as I've moved into hypersonic reading speed, in which my consciousness of individual characters and ability to emotionally relate to the plot line has been decreased to the point of near non-existence.
I've recently been contemplating a re-reading of LOTR, but feel so far away from the potential to immerse myself in the universe that I have not even given it a serious try. Virtually all of my consumption is rapid reading of relatively light fare that I can consume in a one or two sittings.
The introduction of digital means has not helped me (i.e. Kindle, Kindle App), as this increases my propensity for speed, and virtually the only novels which conform to my expectations are overly long novels (i.e. Neal Stephenson) which provide enough change of pace without any particularly deep or moving characters.
Frankly, I have no idea what to do either. I also feel like a greyhound on steroids, incapable of fictional engagement if it does not come at breakneck speed.