...his ability to put on his costume really, really quickly. He got his shirt, arm stockings and gloves on all in the space of a second! No wonder he used to go by the codename Speedy. (Well, he either put his costume on really fast between that second and third panel, or else he spent a really long time thinking of a retort to Hawkgirl's unfinished statement in the second panel).
(Panel from Justice League of America #27, written by Dwayne McDuffie, pencilled by Ed Benes and inked by any one of four different inkers)