It’s called skim reading. One skims the book one is reading to get the gist and thrust of the book rather than dwell on the detail. It takes little or no time to read the book, then.
Some people like to read their books fast. I don’t, except if I want to read more books in the time I have. Mostly, I don’t like to read fast.
A common conception (misconception?) of skim reading is that you may miss something along the way, say something crucial to understanding the story. I was always aware of this when I skim read. What if I missed getting the story? I doubted skim reading would effectively work.
Of course, those that say they skim read with confidence will say that they understood everything. For the rest of us who are honest, they must be geniuses.
But they are probably lying.
There are a few people who can skim read and do actually understand everything in the book out of a gift or natural ability, reading it as they do at the speed of knots. They are the true geniuses. They gather the content and understand the context quickly, whereas others take their time.
To skim read effectively seems to be the domain of the gifted, and there will always be those who say with confidence they skim read with understanding when in reality we haven’t a clue if they really did or not. Good on you if you can.