It probably isn't if you're coming at the material for the first time, or after a long break. I took a 15-week course in Linear Algebra a few months ago, and we only covered about half the book in that time. I spent just about every weekend and evening doing those exercises. It's very time consuming.
However, if you gave me my old College Algebra book, I might be able to skim through it in a month of weekends and refresh myself on most of it, because I've used that stuff a lot in various jobs. Same goes for a lot of (but by no means all) programming books. I'm immersed in that stuff daily, so I can get through it pretty quickly.
It depends. I tend to remember a fair amount of material in math classes from just paying attention. I rarely explicitly study at all for math classes and when I do the results are mixed. Example: In calculus 2 there were all those annoying weird trig integral forms. I couldn't remember them all because it was like a new one arrived daily for a while. I set out to do practice problems with them but still didn't remember much and did poorly on parts of tests that required them. Other things like arc length, double integrals, integration by parts or integrating vertically instead of horizontally, all were just intuitive to me and I remembered them without trying to. I'm not sure how typical or not my results are but in general throughout school if I felt the need to study something and practice it, it was bad news, but if something made sense to me it stuck without much further effort.
I'll add on my own anecdote here, I'm a stats major and I've found that in my stats (as opposed to pure math) classes, I find the learning is very much done just by reading the book because every new concept is just an intuitive application of earlier ones, once introduced to the concept I can understand it even without working through the excercises.
In my more pure math classes (and admittedly in some topics like counting problems) I have to spend a lore more time doing exercises to actually have a grasp of the material. It's frustrating but that's just how life goes.
It can be if you make it. It has been about a decade since I finished coursework for my pure math degree, I do not work in mathematics, and I am enjoying mathematics more than ever just from reading books. I do not miss doing textbook exercises (evaluating integrals? computing determinants? I reach for Maxima or Wolfram Alpha), and am content with reading other people's proofs.
However, if you gave me my old College Algebra book, I might be able to skim through it in a month of weekends and refresh myself on most of it, because I've used that stuff a lot in various jobs. Same goes for a lot of (but by no means all) programming books. I'm immersed in that stuff daily, so I can get through it pretty quickly.