It's actually terrifying to think about. I don't want to denigrate people or be mean, but when you realize that your own standard of literacy is something many people truly struggle to obtain, it helps make a lot of things make sense.
For example, I saw a thread on Reddit the other day where someone mentioned "taking the 30 seconds to read something"; the discussion went on a tangent where someone called themselves a slow reader because it took them a whole minute, and then others chimed in that it was taking them longer still. So, I timed myself and it took me 14 seconds to read it.
What that made abundantly clear to me was that my own experience in the world - seeing _words_ as first class entities everywhere, immediately tagged and understood as whole components, instantly, without ever "reading" them... is not everyone else's experience. Some adults who nominally believe themselves to be literate must actually have to sit there and spend some effort to recognize each letter and sound out a word, the way my children did when they were ~4 (and some children get even younger). What is the world like for these people? I can often speed-read a paragraph and come away with an understanding within seconds, skimming over it for key words and familiar grammatical constructs; my assumption is that most people posting here can do the same. Maybe that's not the case?
There's also a lot of talk on the Internet in the last year or two about inner monologue, with the growing realization that it is likely that a large part of the population simply does not experience the ability to talk to themselves within their own head. This makes me wonder if they can read silently without having to mouth the words; perhaps this is the tip of the iceberg for a differentiation in intelligence that hasn't really been extensively studied yet (unsurprisingly, since psychometrics are not in vogue right now).