Reading fluency is definitely a function of both natural aptitude and experience. Nothing improves fluency likes practice. But for speed reading with comprehension, I suspect natural aptitude is more critical to the upper limit on speed. One of my siblings self-studied a speed reading course while in high school, but didn't raise their speed much--of course, their initial reading speed was about 800 wpm, so that might have been the reason! That sibling also has atypical saccades and near-eidetic memory. One of my children was reading about 700 wpm (I measured) as a middle-elementary student in high-interest literary text, and a little under 400 wpm in low-interest, low-context informational text (it may be faster now, but hasn't been measured recently). DC notes that they do not engage in silent or other subvocalization while reading, with the result that much of DC's vocabulary acquired purely from text does not even have a pronunciation (in their mind) until DC decides to use it in conversation.

I am definitely limited by silent subvocalization, which appears to have capped my reading speed at about 400 wpm, while my siblings read about 800 wpm, with highly accurate retention. On a different, but slightly related note, the reading skill I do have (not, perhaps, particularly useful though!) is that I can read out loud, with good pacing and expression, without paying attention to the content, or while thinking about something else entirely. I've even been known to fall asleep while reading aloud.
_________________________
...pronounced like the long vowel and first letter of the alphabet...