The slow banking system? Don't crypto transactions (on bitcoin or ethereum) take 5-10 minutes to complete, whereas I can tap my credit card to make a transaction more or less instantly?
A transaction on the Ethereum base layer takes 12 seconds to be confirmed once. There is a vast variety of second layer tech available to allow faster transactions.
There are very different amounts of risk associated with accepting a transaction on the base layer of Bitcoin vs Ethereum after n blocks. For example, Coinbase accepts Bitcoin deposits after 3 confirmations (30 minutes) and Ethereum deposits after 35 confirmations (7 minutes).
Compare to traditional banking: Coinbase accepts ACH deposits instantly (up to a limit) and wires of any size can take 24 hours.
Secondly, is Bitcoin + Lightning in combo decentralized in practice?
I don't know. I'm much more interested in Ethereum, personally.
Solana: 0.12 seconds
Bitcoin: 1.2 seconds (on Lightning)
Ethereum: 12.0 seconds
Banking System: ~30 days (2 hours to 5 days for usable funds)
Payment Card Industry: 180 days (2 seconds to a few minutes for usable funds)
15 minutes is 75 confirmations.
The Ethereum network has never deviated by more than 2 confirmations (24 seconds) and with PoS this is even less likely to happen.
Finally, the likelihood of that happening AND a car buyer colluding with block producers to scam you is effectively zero.
You're FAR more likely to be struck by lightning in the 15 seconds it takes the driver to leave than you are to have their transaction reversed on the Eth2 blockchain.