First, let's make this clear. The speed at which you write your first draft has nothing to do with the quality of the end product. It varies by individual. Some can crank out a novel-length draft in two weeks, and others take years. It really doesn't matter.
From talking to others, it seems I'm on the slightly faster end. As long as I'm not in a "muddy" area, I can write 1k in an hour pretty easily. I managed over 50k in the month of November for NaNoWriMo without having to push myself that hard.
Here's something important to remember: My free time is pretty distraction-free (unless I choose some distractions). I have a day job, but once I get home (evenings and weekends), I have no kids to chase nor spouse to feed.
An interesting thing I've noticed during my time in the online world of writers' communities is that there are two polarized types of writers—those who hate drafting, and those who hate editing/revising.
Not everyone falls into one of those two camps. Personally, I like both. I like initial writing, getting the story down, because then I can read it. That's what I do—I write novels I want to read. I think that's part of what pushes my daily word-count along, especially in the final third or so when I really get some momentum going. (Also, by then I'm pretty clear on what'll happen in the rest of the book, i.e., no mud in my path.)
I also like making what I've written even better. Fleshing out some things, tightening others up, hitting things a little harder. Making it all pretty.
How about you? What motivates you to keep moving forward on your work-in-progress? What do you do when you can't seem to get that momentum going?