Complex sentence: He was described as handsome and physically healthy (he was interested in sports), and his mental disability could not be noticed by a brief talk, but it was noticed if someone tried to talk with him for a longer moment.
Split into simpler sentences: He was described as handsome and physically healthy (he was interested in sports). His mental disability was not noticeable in brief conversation, but would become apparent if someone tried to talk with him for a longer moment.