I've always been a fast reader, and 400 pages in a day will fly by for me if a book is good enough. And as long as it's not too dry or abstruse. Or introspective/artsy.
Like this week. Soul Mountain was a tough read but not because it's bad or dry. The book is so peaceful that it's Zen (actually Dao), and that kept lulling me to sleep. I tend to be an insomniac, so I've caught up on my sleep this week--and feel better for it. Still, I have other books I want to get to, and needed to finish this one book to get to them.
That's why I have a back up plan to pick up something else to read for 15 or so minutes to give me a "break" from a difficult/sleep-inducing work. Sometimes it's been a chapter or two from another book on my TBR pile, but sometimes I'll tap a short story, essay, or some poetry. This summer, I decided to make the complete works of Edgar Allan Poe my primary tough-book-filler, and it turned out to be a brilliant choice for that purpose. However, he's so very weird that sometimes I need a break from him as much as I do from the tough books, LOL. Hence the random other works I read as fillers.