Some new cryptocurrencies designed for interbank communication can do 20k transactions per second now, and can scale above that. It's just a matter of choosing the appropriate stack for the task.

Can they do it within ten seconds and completely decentralised, as well? Which ones?

I think the currently fastest is Hedera (HBAR), at 2 minutes per transaction or so, and that it's also the most decentralized, without that mechanism other crypto currencies have that the more you own, the stronger your power over the network. There's still some power at the nodes level though, in that if they all coordinate they can shutdown the network for whatever reason. I also read it's designed such that those metrics improve as the network grows.

But I'm not a specialist, I'm just interested in the technology and play with it a little, so take what I wrote with a grain of salt.

Yes, that was a pretty big dealbreaker... in 2011

Still is one. I think visa processes more transactions now than in 2011.