There are advantages to writing fast.
You have no time to make things up and control yoru characters - you just have to cut them loose - unleash them, if you will, and hope that the story unfolds and develops naturally.
Sometimes it backfires.
A year ago, I started writing a story. I wrote fast - and my characters surprised me a lot. Eventually, my protagonist ended up in seclusion and refused to return to the story. No matter what happened, no matter the threats or incentives, he would not budge. And the story ended sort of like that commercial about haggle-free RRSPs - with this heavy uneasy silence, and a huge unresolved issue.
That's not how I wanted it to end - but I could not bring it to the conclusion that I would have liked without overriding my characters' free will, and forcing them to become something that would be easier to control --- but without a life of their own.
I think that when God created us in Her image, She created us to be storytellers. When we write fast, we become as god-like as we can be. And I hope that when we write fast more often, we become more sovereign, in the true sense of the word.
To many people, sovereignty means controlling every little detail, and ensuring that everything works out just the way you want it. Basically, getting your own way. I think it is Sproul (or maybe McArthur) who says that if there's just one molecule out of control in the Universe, then God is not sovereign, and we have no assurance that good will prevail over evil at the end of time. This is definitely a very human view of sovereignty - based on control, power, and fear of uncertainty.
However - however, I believe that the Divine take on sovereignty is quite different. God doesn't need to control every little detail of the universe in order to ensure the outcome He desires. On the contrary, He unleashes the universe that is wild, unpredictable, and free - and allows it to unfold on its own. He creates the characters without controlling them - no - us! and allows us to run wild and free, make our own choices, and write our own parts of the story.... and still manages to make sure that the story in its entirety is worthwhile, and that goodness and justice prevail in the end. Now THAT is TRUE sovereignty - being able to create a story without controlling one's characters; being able to use characters' own choices, triumphs, mistakes, rebellions, faults, and sacrifices to write the story that encompasses them all in a meaningful way, and ends well.
When we write, we take parts of ourselves and send them out on an adventure. When we write fast, we become less concerned with getting our own way, and more with allowing an authentic story to be born, and a real journey to take place.