Yeah same. I recently read the whole series to my son after he had begged me to stop after a couple of pages about 2 years ago. I told him: just keep your mind open for 100 pages.
The start is so cold, and weird. You can only feel the warmth and beauty that is coming on your second read, and part 1 takes some time to pay-off. What I like about Harry Potter is that many things are not what they seem when later looking back. It's clever like that.
On my own, 20 years ago, I couldn’t stop after starting. Sure the first chapters are a bit boring and different from the body of the books, but I have always given books about 100 pages to grab me, never consciously though about a Volta though, just almost always started to enjoy books after some time.
The follow-ups in a series can be enticing because of part one, but sometimes, as I had with 3 body problem, they disappoint so much or are so different that I just stop.