I'm lucky that writing isn't a race, because I'd come in dead last. I spend forever thinking about each little bit I write. What takes me so long is that I'm mentally living the moments with my main character and then deciding what I need to include to help my readers experience it too. (I'm also working on getting the words to sound right, but I'll leave my perfectionist tendencies out of this, I'm trying to ignore them.)
It struck me yesterday that the perspective-taking I do as a writer isn't so different from what I do as a teacher, especially at the beginning of the year. I think about the school day from a student perspective and the details a student needs to know to succeed (from "Where is the bathroom?" to "What should I do after I finish my work?") Then I decide which ones I need to explain or show them first and which I can leave for later.
The difference is that as a teacher, I need to explain or show pretty much everything. As a writer, I have to be more picky. My explanations need to be more artful and less explicit. Once I'm inside the the perspective of a character, I need to extract the essential and the unique and somehow capture that on the page. That's what takes me a long time.
What slows you down about the writing process?