Some years ago I saw a comms infrastructure upgrade project for a small local retailer chain, connecting all their POS terminals to a proper network. Did they have any plans to implement a smart POS system? Nope, their only reason for quite sizeable investment was to reduce the time to swipe a creditcard by the 5-10 seconds that the old solution (in essence, modems on a phoneline) spent to estabilish the connection to the card processor.

My point? The proper measurement unit for payment latency is seconds, no matter if you're shopping online or at a store.

If it's in minutes or hours, then it might as well be batch-processing of bills with settlement on the next business day.