I think it’s a fantastic idea to write a first draft relatively quickly to get all your ideas out and see if the narrative arc can hold. However, part of writing is editing. You should be editing for months. (First: think big picture, characters, POV. Second: flow, pace, chapter and paragraph structure. Third: detailed copy editing.)
As I’m not a writer–I leave that to the pros!–I’m fascinated by the creative process and love to ask clients and also when I offer representation: how long did it take you to write this?
However, writing too fast can lead to mistakes that are really hard, but not impossible, to fix: pace, inconsistency, weak characters, flimsy motivation. And of course these weaknesses can be evident in slowly written manuscripts too.
There are no concrete rules for the speed of writing; faster is neither better nor worse as everyone has their own patterns to get their book written.
My advice: no matter how fast you write make sure you have spent lots of time outlining the plot and getting to know the characters in your head. Write as quickly or as slowly as your schedule and the muses permit.
But tell me, how long did it take you to write your manuscript?