It has to go out as one, big chunk. Its a lot like a train. And, contrary to popular belief, you cannot change the clock speed on the line. Given a certain amount of traffic, the data should flow through the backbone without delay. At times, when unusually heavy traffic exists, you have what they call congestion.
Congestion is when there is more aggregate data in the network than there is bandwidth.
Well, its like ten roads that turn into a four lane highway. Most of the time you just zip in, no problem. But whenever you've got more than four guys trying to get on the highway at once someone has to slow down. If no-one slows down you have a crash. Now the problem with frame relay is that there is no slowing down.