Credit card tap-to-pay doesn't have to be slow. The transaction could be done asynchronously, and some payment schemes even explicitly classify transit transactions as impossible-to-fail and will ignore things like overdraft limits.

Similarly, it could also be implemented via a rolling payment hold loop. Unattended petrol stations do something similar: you first insert the card to authorize a hold (say $200), then fill up the car, then finalize the transaction - at which point your $50 bill is actually deducted and the hold on the remaining $150 is released. Transit could do the same, with only the very first transaction being slow, and each terminal containing a list of pre-authorized cards.

There are several types of QR code payments getting introduced, so it would be interesting to know the specifics. I'm only aware of using a QR code for Shinkansen tickets, which is something I have used and something a tourist can use as well. And then there's the new tickets slowly getting introduced in some areas, where the paper ticket has a QR code. I've not seen those yet, but it sounds like just an ordinary ticket otherwise.