Using your phone is much slower and there's a significant chance it'll just fail and hold up the person behind you (me).
I wonder if the Tube’s tap in tap out setup contributes to the issues you see.
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
I use my phone exclusively without unlocking it and there's no delay.
There used to be a time where the oyster card was slightly faster than the phone in terms of registering at the barrier but that's improved a lot over time.
I don't live in London anymore but when I've visited it's been so good just tapping your phone through everything.
Additionally the hold ups at the barriers in my experience are more down to people's misunderstanding of the cadence required between each passenger. This applies to both oyster/card and phone, and is more of a UX problem of the barrier rather than the technology used to activate it
A dude was attempting to board, and I was hardly 10 seconds at the scanner when he urged me to get out of his way! It's bad enough when a magstripe pass doesn't scan on the first try... it's going to be murder if everyone's fumbling with their phones and apps and wireless connections.