you can authenticate quite a bit sooner than when the transaction needs to occur..It is easy, but I would argue that Apple Pay use is probably down currently. I use it less since I got the X. It is just something about staring at my phone for a few seconds to make a purchase that seems a bit off right now.
I need more time with it... and more people to adopt it I suppose.
like, you can use applePay while you're standing in line or while the cashier is ringing up the order.. then when it's time to pay, just move the phone to the reader.
point being-- if staring at the phone for a few seconds seems weird to you, it's possible to do the authenticating during a time when you're not doing anything anyway (ie- while the order is being rung up)
(i'm not exactly sure how long until you'd need to re-authenticate.. but there's at least 45 seconds based off my personal experience)