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.
Why do we not see lasting evidence of memorization after the Book of Mormon was published? Doesn’t this mean Joseph would have had to go through the extraordinary task of creating the content on the fly while dictating?