Fittingly, I stumbled on this post by epicblackcar which asks the question: why does writing take so damned long? Because even when you're churning out 4,000 words a day, it still Takes. So. Damned. Long.
Epicblackcar suggests, in his second post on the topic, that our primary failing is structure. If we knew exactly what we were going to write, we would write it. But we're more like construction workers who gleefully build a set of stairs, and then realize it leads nowhere. (I encourage you to read his post. It's epic.) I agree that a lot of the slowness comes from the disability we all have to keep the contents of an entire novel in your head all at once. And the fact that you can certainly get carried away.
Anyway, back to my 2,000 words.