Short fiction is not always quicker than novels for some of us. And I don't think the amount of time you spend on something--anything--determines whether or not it is publishable.
Well, within reason, I'd argue that just about anyone can write a short story or a novel just about as fast as they really
want to write it. No one takes months to write a short story unless it's intentional.
Really slow is definitely not better than fast, but from my experience, reasonably fast is almost always better than really slow. The more time you spend on a short story or a novel, the fewer short stories and novels you'll be able to write, which means the less practice you'll get. And there just are not many selling short story writers out there who take an inordinate amount of time to write a story. Most are very, very fast.
The slower you write, the more natural talent you need to write something publishable. If you spend months on a single short story, it'll take years and years to write enough short stories to matter.
Few writers begin the writing life writing well enough to sell. I'm not sure how many short stories the average writer must write before selling one, but I've seen numbers ranging from fifty to one hundred.
But even if the number is twenty-five, and it takes you six months to write a short story, it's going to be twelve and a half years before you write one that sells. Twelve and a half years to make one little sale.
I know writers write at different rates, and not everyone can write a publishable short story in four hours, or a publishable novel in a month, but let's get real. If you're actually writing, which means planting your butt and pressing keys, you have to press them unbelievably slow to make writing an average length short story take a month.
I'd argue strongly that
anyone should be able to write five words per minute. This gives you a 4,000 word short story in just over thirteen hours, which mean less than two weeks at a mere hour per day, or a 100K novel in under a year, at that same hour per day.
E. L. Doctorow probably said it best,
Planning to write is not writing. Outlining, researching, talking to people about what you're doing, none of that is writing. Writing is writing.