I really liked Cage of Souls, it was like a Jack Vance revival. And it's always fun to reach Tchaikovsky and Alastair Reynolds books back to back.
It certainly varies with genre, and the short writing periods tend to be more common in genres where people write series and the genre has very specific expectations (e.g. romance, crime mysteries) and where huge levels of originality isn't needed. NOT suggesting that is the case for Tchaikovsky.
I've written two novels, and when I first get myself to sit down and write (that's the hard part), I fairly consistently write 2k words an hour. A 200 page novel is in the 60k-65k word range, so 30-33 hours of writing.
If I could get myself to sit down and just write more consistently, I could churn out a lot too (of course whether it'd be good enough and/or commercial enough is another matter - most authors sell peanuts).
That ability to make themselves sit down and write with some degree of consistency is the most impressive part to me with authors who produce a lot, but that probably reflects what I personally find hardest (my second novel took three weeks from synopsis to first draft, and another four of editing; I'm now two years and 40k words into my third novel despite having planned the plot out in detail)
I've wondered the same thing about Stephen King. I figured it'd be easy enough for him to put out outlines and then make edits after others he trusted to write in his style wrote most of the words.
He’s just actually that prolific.
Also Dogs of War was good.