The point made above about Marshals that actually Marshal hits the nail on the head. Ive been on both sides of the equation---been in groups that hold up play as well as being behind others that are slowing things down and to be honest I cant remember the last time Ive had a Marshal do anything about either situation.
Courses obviously dont want to anger the patrons by nagging them to get going, but by not doing that they are angering countless other groups out there. Tell them to play ready golf, dont mark scorecards near the green, accept lost balls, enforce a wave-up on tougher par 3s or force them to let groups go through--there are many options to keep things moving somewhat quickly out there. Id get more golf in if it didnt take me 6-6 1/2 hrs round trip from the house to get a round in on the weekends.