and how long will pass before visa/mastercard start loaning your bitcoins out to other people without telling you? (exactly like commercial banks do today with fractional reserve lending).
What's that got to do with processing times ?
Queues at supermarket checkouts are a problem that needs solved. It will be solved independently of whether payment processors do fractional reserve banking or not.
I don't agree with you that these concepts are "coupled" in the way that you suggest. Anyway, what you are alluding to is something called "leverage". Leverage has been around since the age of the Pyramids and won't be disappearing with the arrival of crypto currencies. It's not an option that's unique to payment processors - anyone who holds any money of any type can engage in leverage and, as I say, it has nothing to do with payment processing systems who's job it is to facilitate the functionality needed at the point of sale.
There's already a grocery store in Vancouver that accepts zero-confirm transactions. I doubt it's the only one in the world. We also have restaurants, pubs and coffee shops that accept zero-confirm on-chain transactions too. I already find paying with bitcoin less annoying than using my debit card.
That doesn't change the fact that blockchain transactions are not scaleable at the point of sale. If any physical retailer of any significant size (e.g. a supermarket) had to do blockchain transactions at the point of sale, their operations would grind to an immediate halt. It's just not going to happen. I develop e-Commerce systems for a living and payment processors are essential to scaling any kind of retail operation. Consider the following scenario:
[1] - you are in a supermarket queue with 30 items
[2] - the cashier rings through your items
[3] - they ask you if you want "cashback" with your sale and if you have a points card
[4] - you pay for the transaction, receive £50 (or $50) cash, which gets charged to your card account along with the groceries
[5] - as you pack your items, the guy behind you realises that the cashier accidentally rang through one one of his items on your side of belt splitter
[6] - the cashier discounts your sale, refunding you the excess balance and asks you if you want the refund in cash or refinded to your charge account
[7] - you leave the chop, having storecard points added, received your "cashback" amount in fiat, and having the corrected amount charged to your account
Now, there isn't a snowballs chance in hell of all that happening on the blockchain in realtime. It's not what the blockchain was designed for. This is what payment processors are for - they handle all this stuff effortlessly and provide added value to the retailer at the same time, for example by insuring the transaction, supporting auxiliary products such as discounts and store cards etc.
So don't have any illusions that Visa and Mastercard are going to loose any business to Bitcoin. They will be needed more than ever if cryptos take off (which I think they will). In fact they will actually be part of what drives the crypto economy and if we don't get them on our side then there will be no "gateway" for big retailers to come on board.