Not just routing. When a plane or truck arrives and is unloaded, thousands of packages have to be scanned, which triggers a cascade of messages to different systems. In addition to the routing and whatnot, customs declarations might have to be sent, track and trace gets updates (both from arrival scan and customs), there's billing etc. All this flurry of activity happens within an hour or so after the plane lands, and then it quiets down until the next plane or truck.

I could easily see FedEx can save money by dynamically scaling capacity to track the arrival of their planes or trucks.