I THINK I FINALLY STREAMLINED MY PROCESS:
First chapter I wrote for my dissertation was about 60 pages (it's shorter now), and took me about 10 months to write. First chapters are terrible and take forever. You don't exactly know what it is you're looking for, and you have no very clear idea what to do once you find it.
Second chapter I wrote was 50 pages, and took me about 7 months.
Third chapter, also about 50 pages, took slightly over a month.
Perhaps surprisingly, the faster I write my chapters, the more I like them.