Could it be that they aren’t getting to “book shop” often enough? Here’s one article about that topic. Note the suggests for letting kids book shop for good fit books. Especially note the next to last paragraph.
https://www.thedailycafe.com/articles/3-ideas-for-book-shopping-during-daily-5
Here’s an example from a first grade class.
https://www.thedailycafe.com/articles/shopping-for-good-fit-books
Kids need to know that it’s OK to abandon a book, as well. If a book requires such a long time to get through, it usually indicates that it’s either not a good fit, or not of true interest to the child.
I’d say let the child try the book they want to read–and review the 3 Ways to Read a Book. Perhaps they’ll realize that it’s not a perfect fit for reading all the text, but that they can learn about the story/fact by browsing the illustrations, charts and other text features.