I'll add my stories to the 2 brother comments, in my prepa my best friend was by far the best in class. He did the 2 year program in chemistry in the first 2 months by himself. He arrived once in exams after a full night of re-reading the 3 LOTR books and still nailed the best grade effortlessly. And to his own admission, he was far from the most productive / smarter guys he knew.

I remember listening to him and a teacher, the teacher would give some anecdotes on what he's seen at the ENS. People reciting world chess championship games at night to fall asleep. Reading massive math books (1000+ pages) in an afternoon. Another story I heard in a different context from another teacher, math students were assigned a python project, after an introductory course. They never did any programming before. One student wrote 300 LoC in a single function and showed it to the teacher. It didn't have a main function, so the teacher asked him how he tested it. The student didn't know what he was talking about. They added what's needed to run the function and it worked.