Hardly supersonic, as fast as blunt weapons and morningstars (which are strictly superior to scourges, apart from not making everyone soil their maille underpants because "omfg, lasherz!").
And the cat-o'-9-tails was actually a relatively harmless disciplinary instrument... think a partially untangled rope (often exactly that on ships). It gets mentioned so often not because it was particularly harsh compared to the altneratives, but rather because it was used very liberally. Something one wouldn't do with implements that'd kill/cripple your men.
Actually, whips make that nice loud cracking noise because, when used properly, the tip breaks the sound barrier when snapped.
The distinction here is between a scourge and a whip. Scourges are a subset of whips, that with multiple tails, which probably would not make cracking sounds.
In relation to DF, the reason whips are deadly is because of their velocity, which can modify the force of an attack. Whips have the highest velocity, listed as 5x (the next highest is flails at 2.5x. most blunts have 2x, edged weapons have less). Whips may not be very big in size, but with a heavy base material the high speed and small contact area allows them to cause serious damage.
Scourges are a different weapon altogether. They have an edged attack with a speed of 2x. Though they are bigger than whips, small contact area with low penetration means they make very shallow piercing wounds. Which isn't to say they can't be deadly. I'd be leery of giving this to the CotG.