Any book worth reading is worth reading slowly.
Now, I can just say, "Did you know PG does that too?".
(We need a name for it.)
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.)
As it is, I find my recollections of the Aubrey-Maturin adventures muddled and vague, since I burned through the first 20 at an addict's pace. Not recommended.
I wish I still had eight and a half more to go!
When I was younger, I read a little Confucius with a lot of time, that's very different life.
You need to come to terms with what the author is claiming, critically evaluate the argument used to support those claims, and then decide the relevance of this new perspective on the world (eg, how will you implement it in your life?)
This all takes time.