Right side
夋 supplies the sound — qūn drifting to jùn through a q/j palatalization shift seen across this phonetic family. Same phonetic gives
俊 handsome,
峻 steep,
竣 finish — all sharing the 'tall, outstanding' shade. That faint flavor carries into
骏: the horse is not just fast but excellent, the word generalizing to 'talented' for people.