I remember reading how the notion that medieval swords were slow, heavy and unwieldy actually comes from a stigma created around the time when rapier duels and fencing among the nobility were commonplace. "Oh, look at those brutish peasants swinging their slow metal clubs around! Rapiers are far more elegant and civilized, I say!" Eventually swords were replaced by guns, but rapier/fencing duels were still a thing, and these 'experts' would carry on the perception that longswords and greatswords were slow (they were slow in comparison to rapiers, of course, but the difference was greatly exaggerated.) This belief would then worm its way into literature and eventually film, and it also certainly doesn't help that most of the swords carried today are either replicas or ceremonial.

TL;DR: The people who created the notion that longswords/greatswords were slow, actually barely knew anything about them and just wanted to hype up their own fighting style.