The Book of Mormon was not necessarily composed by supernatural power. I happen to believe it was composed at least partly with the assistance of demonic inspiration, but it is not such an extraordinarily voluminous book that it could not have been composed by human power alone, as Mormons will often claim.
Last year, I did a 40 day Bible study series centered on the epistle of St. Jude and the themes contained therein. I did all of the work in a single word document. Including passages copied from Scripture, my commentary, commentary from other sources, and the prayer intentions I wrote, the document came out to almost 61,000 words. During this time, I was also working part time, so I only spent about 2-3 hours per day on the Bible study, excluding the time I spent broadcasting it on x spaces.
If by my natural power in 40 days of 2-3 hours of work per day I was able to compose and type 61,000 words, it is by no means impossible by natural power to dictate 270,000 words in 62 full days. I think that if I spent the time I spent typing dictating, I could produce a similar volume of text in two full months of work by my natural power.