Define Performance Diminishing Drug Meaning

That guy smoked pot before the race and raced like a turtle. It was a performance diminishing drug somebody on the other team told him would make him faster.

Someone from the other team told the guy that if he smoked ganja before the race, he would be more clearheaded, faster, stronger, and to his suprise, it took him most of the day to finish. Nobody told him that marijuana was a performance diminishing drug.