Math is slow because you have to work things out as you read it.
Books written very long ago are slow because I read them like a detective, looking not just at what the author means to say, but also at what he's saying implicitly about how things were at the time.
Other books I read slowly because they're so good I don't want them to end. I used to have to make a conscious effort to make Patrick O'Brian novels last, and I stopped reading them at about number 12, to save the rest for later. (I worry though that I wouldn't like them so much now.)
