(maybe a bit clearer comparison if you note that a shopkeeper can make 2 deposits a day because of the fast lines. If it wasn't there, the main queue would take 6 hours to get through (it would get a lot longer, even though the net transaction rate is about the same), and the shopkeeper would be limited to perhaps one transaction a week. As it is, the queue can easily take an hour ;-)
Ok, how about we abandon the use of analogies? This one is just as flawed. If the shopkeeper in your example was like our dumps, he could make 4 deposits a day, 24/6=4. Our dump threads don't need to go and tend a shop inbetween dumps, they can be in the queue constantly.