who’s fast enough with a qr code scanner to actually use it?

It shouldn't take too long to do. You don't need a dedicated app, so you can just double tap the power button to go straight to the camera, scan it, and tap the link. All of that shouldn't take more than about 4 seconds from when you realise you want to do it, if you have your phone within arm's reach.

Of course, I wouldn't be surprised if it takes almost as long as that for a person to make that decision, which means you're looking at at least 10 seconds of QR code before it becomes plausible, which is a very long time in ad space—unless you're expecting people to recognise the ad the second time and plan ahead.