Adler's advice is meant to make your reading more efficient and effective, not to increase the amount of time you spend.

In fact he advocates taking just a short time to quickly size up a book, rather than make the mistake of starting to read a book only to discover halfway in that it is not a good book, or that it doesn't address what you want to know, or whatever else.

He is addressing an audience that will read large numbers of books to learn something, or in preparation for writing, etc.