If Joseph Smith had somehow memorized nearly 10 pages a day during the dictation of the Book of Mormon, we would reasonably expect him to effortlessly quote passages of the book in his discourses.
An analysis by BYU Professor Casey Paul found that Joseph Smith made references to 451 different biblical passages in his Nauvoo discourses, compared to only 22 references to the Book of Mormon. This is a ratio of 21:1 and nothing like what we would expect from someone who had memorized the book or spent his whole life planning it. Joseph seemed to move on from the Book of Mormon once it was published and made nothing of it.
The fact that Joseph Smith didn’t frequently recite passages from the Book of Mormon in his discourses makes it even more unlikely that he had memorized it. This is problematic for alternative theories of the Book of Mormon’s origins because it requires Joseph to have created the content on the fly when dictating.