Mark,
I agree many golfers are very slow, particularly ones on the PGA tour; I will also agree that some courses can be played walking in 3 hrs or less with a twosome and possibly 3hrs or a little more with a 3some and 3:30-45 w/ a 4some.
On certain courses it makes it close to impossible; I am not a slow golfer, I just have noticed how long it actually takes to play golf when walking on certain golf courses. Played with some GCAers at East Lake about a month ago and we finished in 5:00; we waited on a few shots at most, the other guys in our group were not slow and played when ready. It is a big hard golf course; could we have shaved 30 minutes off of our time, maybe, could we have shaved an 1hr, not possible while still attempting to remain competitive.