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Credit card transactions take 3 days to clear the visa network, but swiping your card to pay for something is still instant. These things have already been solved in the application layer.

This is one of the reasons that many Uber drivers in El Salvador cancel rides paid with credit cards. They prefer to be paid in cash.

The mempool is relatively empty to fees are pretty minimal at the moment. But you are mostly right about holding balances on the app on a centralized database. However they are partnered with large lightning payment providers like Strike to make bitcoin transactions over the lightning network to non-government wallets.

They use lightning network.

Lightning Network. [0]

[0] https://lightning.network

They use layer 2 lightning network.