So here's the thing, when I write, I tend to hurtle through the story at top speed, figuring it's better to get the story out while it all makes sense, and then fix whatever problems come up later. And when I say top speed, I mean top speed.
I never learned touch-typing in school, and though I've ben able to learn to type pretty fast on my own, I still write faster in longhand. Though my writing does become less legible the longer I write. So I draft in a notebook, with a pen (and owing to a gimpy wrist, I use fat pens, because those thin little elegant things hurt after a fairly short time).
I prefer cheap dollar store notebooks. Not because I don't appreciate good bookbinding--I do, very much. It's because a cheap notebook isn't precious, so it doesn't matter if I write crap in it. And that freedom to write crap is important. It means I'm not held up by insecurities over the quality of my writing, and I can hurtle through the story and maybe, almost keep up with it.
And that's the real reason I write fast. I seldom plan stories at all, except to have a general idea of their trajectory, and maybe a few scenes I know need to happen somewhere along the way. I write to find out what happens, just like a reader reads, and the faster I write, the sooner I know. And the faster I write, the better chance I have to keep up with the story as it spills out of my brain. because I have this weird notion that if it gets away from me, I might never catch up. I might lose the story. And for me, there's no point in starting a story if I'm not going to finish it.
The surprising thing, even to me and I've been at this a while, is how complete the story is when I get to the end. I always think I'm going to have to go back and add all those subtle details and hints that really tie a story together, but when I go start to edit, I very often find my brain was way ahead of my conscious thoughts, and those things are already there.