I tend to lean toward the idea that all of the problems of a book can be seen in the first 10-30 pages. I know it’s not 100% true, but it’s true often enough that it’s every useful. It’s also why agents often ask to see the first 10-30 pages only.
I remember when I was first submitting and it made me so mad that I got rejections based on 10 pages. It seemed so unfair. I kept thinking that all of the problems they mentioned in the rejection were things that were answered later. And it’s true, they were.
But they should have been answered earlier. In the first 10-30 pages of the book. My pacing was off. My understanding of what the reader wanted/needed to keep reading was off. I just wasn’t there yet.