Know why you're doing it. Why do you want to write a fast draft? Is it because life gets too busy and if you don't finish your draft this month, you may not finish it at all? Is it because you need a challenging goal to keep you motivated?
Those are a couple of reasons, but, for me, here's the main one: Once I have my whole idea down on paper, I can really see the shape of it. It's much easier to revise the story that is on paper, no matter how bad it is, than to revise a story still stuck in my head. Plus, by writing quickly, I forget plenty of it along the way. I'm always whisking forward through plenty of words and character actions and plot points every single day. There's no way I can remember all or even most of them. So when I go back to read my full draft, it's almost like reading someone else's book--I have that much distance and perspective.
The first draft of my first published novel was written in 21 days. The first draft of my second published novel took years. If you read them both, I think you'll agree that they are similar in quality, but I have to say, the 21-day venture was MUCH less painful on my poor little brain.
So think about why you're doing this. If the goal is really just to get the arc of the story down so you can have some perspective and go in with clarity and excitement to revise, then why are spending so much time worrying about the words you are writing and how wrong or bad they may be? Individual words and sentences and paragraphs and even scenes don't matter at this point. Just work your way through the main arc of the story. Don't worry, just keep moving forward to the end!