I find that the most common cause of slow play is the idiot.
He's the guy who has 180 yards to the green, fats a 6-iron about 10 yards, goes back to his cart for a new club, fats that one another 10 yards, looks at his buddies like it's their fault, takes at least 20 practice swings trying to determine the swing flaw, goes back to his cart for another club, repeat, repeat, repeat until you are ready to stick a fork in your eye.
He's also the guy who gets down in an obscene crouch to line up his putts, knowing full well he has absolutely no idea how to read a break but has seen a guy on tv do it this way so what the hell, takes a good four putts to get in the hole, and then pulls out his scorecard and turns to look down the fairway to count his shots. Usually you can hear him say to his buddies, "Damn, that was a rough 6."