Interesting that you also track reading speed. My speed differential between different types of of books always surprises me a bit. Usually it's something like, "Wait, I'm only 20% through this book and I've been reading it for two months?"
For those who don't know, @schmudde runs a blog called Beyond the Frame: https://schmud.de
He also has an excellent Strange Loop talk: https://www.youtube.com/watch?v=SU6NvkkF4Xk
It is funny how much variation there is between books/genres though! The sentence complexity and variety definitely comes into play here.