Most of the layout owners in my area use a 3:1 clock (as do I) and operate an 8 hour shift in 2'40", one uses a 2:1 clock and operates 6 hrs in 3.
The key to using a fast lock is to realize how much a layout can do in a given period of real time. If a yard can only switch 100 cars in a real 3 hr op session then it doesn't matter what the clock speed is, they can only switch 100 cars. If the railroad can only handle running 8 trains in a real 3 hr op session, then it doesn't matter what the fast clock speed is, they can only run 8 trains.
There are also set times that it takes to do things, like classify a train, build a train, switch a town, run across the railroad, etc. They take a set amount of real time to perform those tasks. As long as you allow that relative amount of time in with the fast clock, things will work. If it takes a real 30 min to classify an inbound train at a yard, then with a 2:1 fast clock you have to allow at least an hour, with a 3:1 fast clock at least 1.5 hrs, with a 4:1 fast clock at least 2 hrs and with a 6:1 fast clock at least 3 hrs.
Where people get in trouble with fast clocks is using a prototype timetable and condensing it down to fast clock speed (guaranteed to fail about 99% of the time), just flat running too many trains for the railroad to handle (regardless of the clock speed) or compressing too many tasks into too little of time (running 4 trains into a yard in a fast clock hour when it takes over a fast clock hour to process one). Most times when people say they got rid of the fast clock and things improved, what they really did was run fewer trains. If they had run fewer trains and kept the fast clock it would have been the same improvement.
The fast clock really didn't have anything to do with how congested the railroad was, it was the number of trains or tasks and how close they were placed to each other. All the fast clock really does is regulate the sequence of things. You can have a sequential railroad (run train A, when it finishes then train B, when it finishes then train C, etc.) What a clock (real or fast) does is allow multiple things to happen in multiple sequences.
Another thing that gets people hung up with fast clocks is they get more focused on the clock than running the train. OH MY GOD, I'M LATE. Who cares? Trains run late. With a time table, scheduled trains can be either on time or late. On time is exactly one minute wide. EVERYTHING else is running late. If a train is late the railroad/dispatcher has mechanisms to deal with it. Most layouts I have operated on, the total run time from one end to the other is 10, maybe 15 minutes. Even if I am at one end of the layout and the dispatcher fails to take action and I have to wait for a train leaving the other end of the layout, it's only about 10-15 minutes wait time. No one will die.
When used effectively a fast clock can be a great tool for those wanting more involved operations.