This should probably be a comment, but it's too long. When I was in high school, I played the clarinet, so the sheet music I saw was usually the clarinet part of a piece written (or transcribed) for bands. I had no problem reading and playing that music.
But I've also seen the sheet music used by the conductor of the band, showing what every instrument is playing. That's obviously far more complicated to read.
On the one hand, I couldn't imagine sight-reading a conductor's score in the way I could sight-read a clarinet part (though I suppose experienced conductors develop that ability). On the other hand, studying the score would give the conductor a genuine understanding of the composer's ideas and how they fit together, an understanding that I could never get from the clarinet part alone, and that I only occasionally got after hearing the music several times in rehearsals.
What does this have to do with mathematics? When reading mathematics, one wants to get genuine understanding, analogous to what a conductor gets from the full score. The mathematical analog of the clarinet part would be a very superficial understanding, perhaps enough to do routine exercises but not much more. So it is reasonable to expect that learning mathematics would require work similar to studying a conductor's score. In particular, it's unrealistic to expect to learn mathematics by "sight-reading".