[nly]

It's not the tech per se, it's that you have to open the app or at least unlock the phone (whichever it is), and people are more cautious with their phones compared to a card. They also tend to watch the screen instead of where they're going.

I can slap down my card with my arm outstretched, without breaking my stride, and before I hit the barrier it opens.

Observe a phone user in a London rush hour and it causes a noticeable backlog at gates

[kamilner]

You neither need an app nor do you need to unlock your phone in order to use it for the ticket gates on the tube. Perhaps older phones need to be unlocked?

I use my phone exclusively without unlocking it and there's no delay.