I think the real problem is they don't keep sufficient spare planes ready to roll at airports.  Granted that's an extremely expensive proposition, but even a little extra slack can greatly reduce delay propagation.  As you increase efficiency you can always increase utilization to the point where the system becomes unstable.<p>Edit: I am not suggesting you need a spare at every airport, just that increasing the quantity of spares or increasing scheduling slack time fight delay propagation where increasing efficiency does not.