Dear Rahul,

You might have heard "Practice makes a man perfect”, out of all other reasons this was the major reason why Swami Vivekanand was reading more number of books and not only reading but grasping it word by word.

You can observe this yourself, when you start reading a book after a long break then your speed will very slow but once you are about to the book after reading some 150-200 pages then you will find the your speed has automatically increased.

In those times there were no electronics gadgets to minimize someone's attention spam and Swami Vivekanand was more into meditation and that brought him a good concentration and memory power.

Moreover, Swami Vivekanad was not reading something keeping in mind that he has to use it for achieving materialistic things, else he could have used the shortcuts too.

While he was reading, he used to connect the things and learning. This was helping him to memorize things for long. By this way, you can get every details of what you are reading, and it is the best way to memeorize when you make your own version of story in your mind about what you are reading.

So basically Swami Vivekanand had a focused mind. Such mind is the source of all power. When Swami Vivekanand read a book, it was like the time ceased to exist for him or as if the world died for those few minutes. And then you know that the lesser is the  distraction the more focused state of Mind a person will have.

I hope this answer meets your expectations.

Feel free to connect me in comment section for further discussion regarding this.

Al the very best!