Speed
There is an old saying that goes something like: "You can have it fast, you can have it cheap, you can have it good. Pick any two." Meaning that if you want it fast and cheap, it won't be good; if you want it fast and good, it won't be cheap; and if you want it good and cheap, it won't be fast.
Unfortunately, when it comes to writing, price often drops out of the equation completely, because how much the editor pays is determined in contract negotiations that are generally unrelated to deadline. One thus ends up with "You can have it fast, or you can have it good, but not both." This attitude has been around so long that it has percolated down to the reader level, leading to lots of grumbling when a writer puts out books "too fast." Often, the grumbling gets done by people who haven't even read the book…and who refuse to do so on the grounds that "anything written that fast can't be any good."
As usual with this kind of thing, there is a grain of truth in it that is being blown up into a zeppelin-sized, wrong-headed rule. The grain of truth is this: Every writer has a writing speed that works for them, and trying to push stuff out faster than this speed results in a drop in quality. This does not, however, translate into "writing six books in a year is too fast; they can't be any good" applied to all writers, because the "too fast" production speed varies from writer to writer.
I know professional writers for whom taking less than a year and a half to write a novel is "too fast" - it results in a drop in quality. I also know professional writers for whom writing a book in less than two weeks is "too fast" for the same reason…but taking three weeks makes no difference in quality that I can find. Yet the slower writers are admired, while the faster ones are castigated for scrimping on quality.
The really odd thing is that the folks who think that three weeks is "too fast to write a good novel" are often the very same people who proclaim that quality work comes through inspiration - and that when one is inspired, one can sit and write golden sentences for hours on end without effort (though they'll allow the writer to complain of cramps in their hands at the end). Apparently, writers are not allowed to be inspired for an entire novel's-worth of material at once, and inspiration is supposed to take time off between chapters and novels so that their publication dates will be properly spaced.
There's another problem that arises when critics, reviewers, and the general reading public make judgments of quality based on the perceived speed of writing, and that is that the number of books a writer has coming out in any given year does not necessarily have anything much to do with how fast those books got written. It can take a long time for a book to work its way through the whole publication cycle, even for a much-published author.
When one works steadily at a book-per-year pace, one can easily end up with three or four unpublished novels in the pipeline. If one is delayed (there's a printer's strike, the cover artist was backed up, there were three other books with a similar theme coming out that year and the publisher pushed it back to avoid competition) and one is rushed forward (there was a sudden gap in the schedule because someone else didn't deliver on time, and this book was done), one can easily end up with three titles coming out in the same year.
If the writer has been working on spec (that's "on speculation" for freelance fiction writers, not "to specification" as it would be for freelance article writers), and has had to submit a series project a couple of times before it sells, it's easy to end up with even more new titles coming out in one year. And then there are those "trunk stories" - the ones written ages ago that just didn't find a market, and that have been sitting in a trunk (real or metaphorical) for years until a random conversation with an editor suddenly results in a sale. On occasion, I've heard readers complaining about a "too fast" writer because they didn't realize that the books they were complaining about were a big chunk of the writer's backlist that had been written and published years before, and were being reissued to a new audience.
And then there are the books that the writer has been thinking about, and sometimes researching, for years or even decades before sitting down to put them on paper in a white-hot rush. Again, the assumption seems to be that no one could possibly work on more than one novel or story at a time, even though author's papers are freqently littered with bits and pieces of not-yet-written stories, partial manuscripts, and various other scraps that were obviously produced at a time when the author was supposedly concentrating on some other, now finished, project.
What this means is that readers can't tell how long it took the author to write a book. There's not much point in explaining all this to them, though it can be fun to mention (if you know it) that the literary masterpiece about which someone is currently waxing lyrical took a grand total of six weeks to write from the first typing of "Chapter One" to "The End." The point is that if you happen to be a really fast writer, don't worry about it…and don't let anyone tell you that you have to slow down in order to write well. If you happen to be really slow, the same caveat applies in reverse: Don't let people tell you that you have to speed up. There is no One-Size-Fits-All process. Figure out what works for you, and then keep doing that.