I was a school librarian when the later books were published and there was always a lot of competition to borrow them, but I always used to talk to the kids about the books when they brought them back, and you could tell that a lot of them, especially the younger ones, hadn't actually read it, or properly understood it.
I can see two sides of this. I suspect during the Harry Potter craze, as you say, competition was part of it and some children just wanted to 'read' HP because they'd heard of it.
But I don't understand what is bad about reading a book and not understanding all of it? I had an older brother who was precocious (I wasn't) and a mum who loved reading aloud, and so I listened to whatever my mum was reading aloud with him, and that way I got a lot of stories when I was far 'too young to understand'.
I don't think it was bad for me. I think the opposite. I got to grow up slowly with those stories, re-reading them and learning more. I think it's actually what made me a keen reader - because I was never scared of not understanding a book first time round, and it never unsettled me when I came across words I didn't know or a description I couldn't follow.
So when, much later, I came to read things like Shakespeare for school, it felt fine - it never bothered me that I wasn't really grasping 'all' of Hamlet or 'all' of Pride and Prejudice.
I still feel like that. I'm always going to come across books I need to re-read.