Almost ALL design features slow down play if you jam too many people on the course at one time. Looking for balls slows play; raking bunkers; lining up putts; etc etc. If we'd all just build boring golf courses, they wouldn't be so busy, and then play would move along just fine.
Slow golfers also slow down play. It's funny how Scottish links have a lot of long grass, deep bunkers, and sometimes large, undulating greens, and if there are no Americans in the way, they still play in three hours.