How long did chapter 17 take you? To me that felt like the longest chapter. Although I did not really time myself on either chapter 17 or 19. It could be that I enjoyed chapter 19 more, so my perception of it didn't feel as long as chapter 17. I loathed chapter 17 by the time I was done with it. I didn't have that feeling at the end of 19.
Let's see. I have been playing long enough that the first time I finished all the content and had to wait for a new chapter was after chapter 14. Chapters 15, 16, 17, and 18 took between 3 and 4 months each, so chapter 19 taking about 5 months had me worrying I would not get it finished before chapter 20 was added. Little did I know that the chapter 20 release would be delayed. lol
But anyway, that couple of months in between chapters let me get everything prepped for each new chapter release, so I could dive right in and saved overall completion time because of that. This free time also helped stockpile PPs and time boosters, of course, which also sped things up.
And for a more recent comparison, my Beta city went from chapter 1 to chapter 14 in 14 months, before I started slowing down there.