At a mere (say) 240mph, 5 miles is 75 seconds. Talk to an experienced pilot about how short a time that is, when a bottom-10% pilot is trying to figure out some problem with his instrumentation, or has set his radio to the wrong frequency, or whatever.
100mph is probably around the top speed for home built FPV quadcopters with 5 inch propellers; that's neither uncommon or expensive. Amazon's drones are probably built for efficiency though so they will go slower than that.
5 miles at 50mph would give only Amazon 360 seconds to fix the bug that caused the first drone to crash. Or figure things out enough to get the second drone into manual override mode.
