For example, suppose you can copy 5 pages in 25 minutes. If the textbook is 300 pages, you finish copying the whole page after 25 hours.
Looking at this estimate, you might lose motivation thinking that you have to spend 25 hours. However, that is a misunderstanding. You do not have to spend 25 hours.
Shakyo is like an auxiliary wheel until you can ride a bicycle. The state that you do not understand the book is the same as not being able to proceed forward just by falling on a bicycle. You can move forward by attaching an auxiliary wheel. While you are running with auxiliary wheels, your skill to ride bicycle improves steadily, and eventually, you can ride a bicycle without auxiliary wheels.
Consider the worst case. It is a case where you could not learn efficiently. Even then, you achieve a feat of copying a textbook in 25 hours. There are concrete goals to achieve.
There is no definite goal in the state you think you should read a book that you can not understand. By clarifying the goal, it makes it easier to maintain motivation. Shakyo is a means, not a purpose. The purpose is to advance learning.
Moreover, instead of the goal after 25 hours, set the goal after 25 minutes first. We learned it in “The tutorial is near the goal,” the effect of bringing the goal closer. By doing Shakyo, you are going to be able to learn more efficiently. Someday you do not feel the necessity of Shakyo, and you realize why I compare Shakyo as an auxiliary wheel. You can remove it at the timing when you no longer need it anymore.