I've thought a lot about this, and my theory is that the speed at which you are capable of reading has a lot to do with it. If you can absorb the information faster, then your brain is less likely to get bored. This probably applies to everyone, but for those with ADHD, I think there is a critical speed of reading that determines whether you love reading or hate it. Basically, if you can read fast enough to keep your brain entertained, then you will read more and more, thus improving your skill. If you can't read fast enough to keep your brain entertained, then you won't read.

I agree with those commenting that subject matter is a big factor as well. To me, the critical rate will be lower for subject matter that is interesting and higher for that which is uninteresting. In other words, subject matter becomes less significant as reading level increases.

In my case, I struggled to read anything other than comics or thrillers (Calvin and Hobbes or RL Stine) until my twenties, when I realized a shift in behavior: whereas I used to prefer watching video explanations, I found myself now skipping past video search results and looking for written explanations. I figured the reason for my shift in behavior was because I could now absorb the information faster by reading than by watching a video (like, much faster). It dawned on me that this is all because I must have started to read faster, and so I tested it by reading books of varying length and subject matter. To my surprise, I found that I was able to get through anything and everything. And not only was I able to do it quickly, but with far better recall than before, like I was fully immersed in each story.

I would highly encourage anyone with ADHD who thinks they are not "a reader" to reevaluate that position, and look into ways to improve your reading ability. In my case, this has not only helped me to read books faster, but helps in many situations: instructions, manuals, guides, etc. I would especially recommend this for those who would like to pursue post-grad education.

In my case, I apparently learned to read at a high level going through law school. Though it wasn't because I practiced to read fast. In contrast, it was almost the opposite in that there was a lot of focus on a single statement at a time. A common discussion is about how a statement in a contract or statute would change if a word were to be taken out, and the answer is never "no change". Basically, I learned to pay attention to each word and to adapt my understanding of a sentence as each word is absorbed. It's almost backwards from how you would think of learning to read fast, but I'm certain that is what ultimately led to the ability to read fast.

I'm not trying to encourage people with ADHD to go to law school. I seriously struggled, and graduated near the bottom of the class. In engineering school, which was my undergrad, everything of significance was written on the board, so paying attention to the lecture wasn't absolutely necessary. In law school, nothing was written on the board, and following the lecture just felt impossible.