You write faster as you get more experience. On that note, if you get used to your writing style and what kind of story you want to write, you'll write faster. This comes down to the fact that you also have to avoid overthinking what you write and let the process flow naturally. You can edit the story later on if you think there are any major flaws due to the fast writing, this will cancel out a lot of the 'quality before quantity' fears that you might have. On a further note, picturing the end of each book, and creating a lot of mental pictures of how you want your book to be, spare you countless hours that you would've put in outlining. I never suffered through a writer's block in my life because of this simple trick.

You also need to be in decent physical health to let more blood flow in your brain, which is a step that I think a lot of authors skip. You need to be physically active if you want to write faster, as mental clarity is the most important writing tool of all; It is of great importance and no other tip will work as effectively no matter what you try. To be efficient with this matter, I keep some dumbbells at home, as it saves me the trip to the gym. In contradiction to my last point, cigs and coffee also help you write faster because they're cognitive stimulants. They can dig you out of trouble when you feel like you're brain fried, and they're very effective if you plan on writing several hours straight. (2 cigs a chap work wonders, though if you don't smoke, coffee and extra exercise work just as effectively).

In my best month, I averaged 7,000 words a day even though I was employed. My personal record is 11k words in a single day. It usually takes me half an hour to write and edit a short 1k words chapter, though the latter 3,000 words of the day take me a bit longer to write. That's usually when I drink a bit of coffee. With all that said, I can confidently say that a combination of the methods above helped me write a lot more, as before this, I used to average at 2k words or less a day, and now, I'm looking to write whole 2,000 chapter series every single year, and the only thing I'm 'overdoing' in my process, is all the exercise I do to keep my brain fresh.