Well, if you are an adult and reading in your native language and don't have any kind of cognitive limitations I don't see why "reading comprehension" is even a topic. I have only ever heard that talked about when studying a foreign language and moving from lessons and textbooks to regular books.

And of course your mind wanders and you find a book "hard" if you don't actually like the story. Happened to me more and more with the basic thriller, which is why I took a break from the genre. And these are definitely not "hard" books.