I big factor is that people just need to have self awareness as well as situational awareness. I watched a lot of golf on TV before I started playing, so I thought thats how slow everything went on every course. I always felt rushed, and would let everyone pass just because I wanted to take my time. After a while I just started to pick up the pace when people were nearby. If they are right on my group for a hole or 2 then I will let them pass, but most of the time I can just speed up if I think I am holding another group up.
Also it really matters on my group because if I am with certain people we can finish a round in 3.5 hours, but with others it will easily be close to 4.5. I think it goes along with the ego thing to not want to put away driver and make the better play just in case you kill one. I have friends who will go through a whole box of balls just on tee shots, and that just kills the pace.