I think it's more useful to think in terms of hours than weeks or days or months.
When I had a day job, it took a lot longer to finish a book, but that was because I only wrote for two hours a day. Also, I have books that took me a year to write, but if I'm honest, they really took me three months to write, because I worked on them for a month, and then couldn't bear the sight of them for nine months, and then got motivated and finished them up in two months. I suspect this is similar for lots of people.
I have had one big jump in productivity, and that was to go from 1K an hour to 2K an hour. This happened after I learned Dvorak (for carpal tunnel. It didn't work. I'm fairly sure I have Reynaud's and that causes swelling/pain in my hands. Best trick for me has been heating pads in fingerless gloves.)
So, I'm twice as fast now thanks to the keyboard layout, but beyond that, it still takes me the same number of hours to write a book as it used to. It just depends on whether they are spread out or not.
I really think a big component of writing "faster" is really writing "more often" or "more hours a day." Sometimes you have those hours. Sometimes you plain don't. And there's no shame in that. It is what it is.
