I've seen players play slower in carts. It happens all the time in front of me.
Moe and Larry share a cart, drive to Moe's ball, Moe gets out, dawdles choosing a club, checks the wind, takes his practice swings (usually three, two if I'm lucky), finally hits his shot, cleans his club, replaces his divot (again, if I'm lucky), puts his club in the bag and gets back into the cart.
All this time, Larry has been sitting in the cart, five feet away, pondering the meaning of life or what he's going to have for lunch at the turn. They drive twenty yards to Larry's ball, and Moe waits for Larry to go through the same routine.
If I could have one wish granted per round, it would usually be for Moe and Larry to split up and walk -- or drive -- to their ball while the other guy is playing his shot.
Now that would save 45 minutes to an hour.