I'll second this. My reading speed varies based on the "density" of the text, for lack of a better term. I can blow through a Jim Butcher or a Ben Bova novel in a matter of hours. Granted, my information absorption isn't complete and in subsequent readings I pick up on foreshadowing I didn't notice the first go through. Denser fiction ("A Farewell to Arms") or anything translated into English ("Three Body Problem", "All Quiet on the Western Front")* requires me to slow down and think about the context to understand what's going on.
My speed reading with non-fiction is similar. I can burn through modern non-fiction in a matter of hours or days, depending on the length of the text. Reading Greek philosophy, like Epictetus' "Discourses" takes significantly longer, weeks or months sometimes.
*Shakespeare occupies a weird middle ground, as my difficulty with that is mostly translating the vernacular into modern English in my head as I read along.