It took Joseph Smith around 60 working days to dictate the Book of Mormon but we know the translation was a stop-stop effort between 7th April to the 30th June. The whole process would have been quite strenuous working each day having to dictate the text, write the words down, repeating them back and confirming before continuing.
John Welch and his wife attempted to replicate the work on a small scale and noted that:
Altogether, our results showed empirically that a translation rate of right around 20 words per minute was quite possible. But we couldn’t imagine sustaining that rate hour after hour, day after day. Our hands got tired, and the one playing Joseph needed to catch his or her breath and clear his or her voice. We used ballpoint pens. We imagined Oliver dipping and using his quill pen.
…Although not strictly scientific, this exercise produced a flood of experiential insights. The stress of trying to achieve a maximum accuracy took a substantial toll on us.– Timing the Translation of the Book of Mormon “Days [and Hours] Never to Be Forgotten” John W. Welch
If Joseph Smith had written the Book of Mormon previously. it seems far-fetched that he would have gone through the exhausting translation process just to make it appear as if he was receiving revelation. Isn’t Joseph’s persistence evidence that (at least he thought) it was a real translation?