if I have it right the maximum number of transactions btc network can handle is 7 per sec. coffee purchase and walmart trip has to go offchain eventually.
Only temporary limit while devs still work on improving the system. It's still in beta, and they don't want it to explode in use before its ready. Eventually, transaction limits will be removed, blockchain will be pruned, and the system will be able to handle way more transactions per second than VISA or anyone else (it can already handle such volume with current technology, but we don't want to force people to download 500MB blocks just yet
)