Most people don’t measure or worry about the speed of the first shot in a double-tap. Taking three seconds to align the gun, stabilize your grip, and carefully aim before hammering the trigger twice is counterproductive.
If you watch, you’ll frequently see this on public ranges. A shooter will start from the low ready position, get the gun up on target, and then take forever prepping the trigger for the first shot…and then slamming the trigger for the second shot, regardless of where the gun is. That’s how indoor ranges end up with bullet holes in the roof, and target carriers that don’t work so well.
It’s extremely important to differentiate the “Amateur Hour Double-Tap (AHDT)” from controlled pairs and hammers. A controlled pair is two precise shots with two flash sight pictures, and a hammer is a pair of controlled shots with one sight picture. The difference is that you’re never out of control of the gun; unlike the AHDT where the shooter really isn’t controlling the gun after the first round leaves the muzzle.
I also agree that training to shoot two rounds and then stop on a target is folly for a defensive shooter – as discussed in the Stopping Power post, you have no idea how those two rounds are going to affect the target. That’s why places like Gunsite teach the “non-standard response” drill, where you shoot the target until it goes away. In all honesty, the only people that have any business practicing double taps/controlled pairs/hammers for any significant amount of time are competition shooters, where the “two shots on a target” is doctrine. In fact, it’s so deeply ingrained in my head that I’ll find myself doing every kind of pistol practice in groups of two.