A normal backcast does not exceed 100 ft/sec. and takes about one second to unroll. You can easily follow the fly with you eye. You can also create that crack at will by changing the timing of when you start your forward cast. The crack you hear is due to your changing the direction of the fly in a minute fraction of a second.

Since the speed required to produce a crack is greater than 700 ft/sec, the interesting question is, "For what distance is the fly traveling at a speed of more than 200 ft/sec.?"