I had heard about this feature and set it up before a recent trip to London. I have an Oyster card and I used both my phone and the Oyster card on alternate days to test it out. The phone was just as fast as the Oyster card. I can confirm that daily cap was applied when using the phone.
Basically what transit express does is eliminate the need for you to wake up your phone and login to Apple wallet. Just tap your phone on the pad. I noticed that my phone registered the tap even before it made physical contact with the pad. Worked well enough that I’ll probably turn in my Oyster card. One less thing to remember to bring and carry around.
More info about it here.
More about where it can be used.
Hopefully there is a similar feature for android users? Maybe via Samsung pay?