For me, personally, my goal is to maximize the amount of knowledge and understanding I can acquire per unit of time. As part of this I guess I'm testing myself to see how long I can focus for each day I devote to studying, and to see if once I've quantified it, if I can beat it.

I guess after thinking more on this, I'm not so much focused on "how many pages can I read per day" as I am "how long can I stay focused and study per day"

Also, since I wrote this I've started using spaced repetition flashcards (with Anki) to better retain details as I read through books. I've found that spaced repetition flashcards let me relax a bit while I'm reading because I don't feel like it's so important that I immediately remember everything as soon as I read it since I'll be revisiting it later. This seems to work much better than just taking notes.