Writing did get somewhat harder when I transitioned from part 1 (the Java interpreter) to part 2 (the C VM). C is a more finicky language in a lot of ways. Things like the order of declarations matter. So figuring out which order to introduce snippets of code within a single chapter tends to take a little longer, but it's not a big difference.
My rate of progress has generally slowed over the past few years, for a lot of reasons. My initial enthusiasm has naturally waned. There isn't much novelty left in the project for me — writing each chapter is much like writing the previous one. So it's mostly an exercise in discipline now. Meanwhile, of course, I keep stumbling into other new things that I am excited about, so staying focused is hard.
The last few years have also been personally difficult for me in a number of ways. My mother got cancer (in remission now!), two family members died, two friends died, my dog (the one on the back cover of my previous book) was in ill health for a long time and then had to be put down. Lots of other personal stuff like that. 2018 was one of the worst years of my life, even though I'm still very fortunate in many ways.
But through all of that, I've managed to keep going. I work on the book every day, so I worked on it in the hospital waiting room while my mom got a CAT scan. I wrote a measly 59 words the night after we put down my dog. I wrote in the hotel before my friend's memorial service and on an airplane on the way back from my aunt's funeral.
In some ways, the project is a constant comfort in my life, and it's also taught me a lot about the power of perserverance and discipline. I'm really looking forward to it being done and taking a long break. But I'm also looking forward to seeing what I'll be able to do with this discipline next.