I write very fast. My collection of short stories is 15 pages, single-spaced, so far. Sure, I can go back and pick at it a bit, but the quality would not be improved if I wrote slower.
At the same time, everything I write is based on ideas that I have always had in my mind. The ideas have been accumulating for a very long time, and I have turned them over in my head repeatedly, over the course of several years, in some cases. Any sense of speed, then, occurs in the context of several decades.