I went through an Optimization course last semester (CS, grad), so it doesn't really qualify as an "out of class experience", nevertheless reading it was quite optional, and, actually, the questions I asked myself were very similar to yours.

Especially in the light of those small remarks textbooks tend to make along the lines of "we don't have any more space here, so if you're interested, the excellent book by X and Y is a very nice read". As if they were referring to some light and entertaining book if the one you were holding weren't really enough to fill up your entire afternoon.

Instead, I spent hours on the part about Conjugate Gradients for example, coming up with different (mostly wrong) mental models, drawing various maps, and thinking about what's wrong with the way I try to study math. (I also ended up at #lesswrong, asking people how they study. Also brought home some ideas.)

So, in the second half of the semester, I upgraded my method to the proposed-by-some-people "ignore all the proofs, and generally, all of the textbook, try to complete the excercises that are likely to come up on the exam, and don't try to see everything". Which kind of worked: I understand most of the concepts, I can solve actual problems, and also, passed the exam. (As if that one counts as a proof of knowledge...)

But I'm still curious how studying textbooks is supposed to work. Like...

• what is the goal of people when reading textbooks? being able to solve real-world problems? passing exams? solving all the excercises? getting the warm fuzzy feeling of having eaten a huge book with lots of formulas while getting that "yes I understand" feeling that may or may not be the same as really understanding stuff?
• what is the goal of people who write textbooks? is the fact that they are hard to read an unavoidable thing, a way-too-common flaw or... are there people who read math like I read MLP fanfics? Is it possible to fix this?
• and also, the statistics you mention. About the average WPM when reading math books...