They do delay customers by 20 seconds, because if we’re carrying metal then we’ve got to be inspected by security.

Capchas hold you up 20 seconds every single time, whereas metal detectors usually don't alarm at all.

Wasted time is still >20 seconds every time you encounter a metal detector, unless you really don’t ever carry or wear metal. Bonus points if you regrettably have a bag that needs to be searched.

It takes me 20 seconds just to be sure I've accounted for any possible metal in my pockets.