How the heck does Brandon Sanderson write so fast?

Like...how!?

This man can write multiple books and release them all in the same year.

From what I observed from Mistborn, his prose is very simple to the point it's invisible, so he can focus on the story more, which is fine with me, and I find that good. Does he sacrifice a few aspects in exchange for speed? Is that the secret? I mean, his prose is way simpler than RR Martin's and Curtis Jobling's.

Or is there more to it? How do you think he writes so damn fast?