If they’re all based on blockchain, won’t this hugely slow down transaction speed? Right now paying by visa involves a tiny message to my bank where it goes “ye

If they’re all based on blockchain, won’t this hugely slow down transaction speed? Right now paying by visa involves a tiny message to my bank where it goes “yeah ok” to the sale. One would think if they’re going to invoke a digital ledger, then this will have to be computed on every single transaction. I can’t see that being done quickly on billions of transactions a day.