Being a hack doesn't necessarily lead to slow play, and vice versa. The problem is that most of those guys have zero clue how to play ready golf. How many times have you seen this scenario play out?
Two idiots are cart partners. Idiot #1 is in the right rough 150 yards out, Idiot #2 is in the left rough the same distance. Instead of one guy grabbing a club or two and heading to his ball while the other takes the cart, both of them sit in the cart while one plays, then both hop back in and take the cart over to the other ball while the other plays. Rinse, repeat down the entire fairway.
Same thing on the green, each guy has to wait into the other guy is completely finished before reading his putt - apparently it's too much to ask to read your putt while the other guy is putting.
I've played with speedy 20s and glacial 5s. It just a matter of knowing how to be ready when it's your turn, however many turns that may be.