I take your point that you wouldn't want to open source everything, especially not the few critically secure bits of your infrastructure.

That said, money transfers take a couple of days because most interbank clearing houses were modelled after paper-based batch processes where people shuffle through paper a few times a day.

There are several real-time payment schemes in the world. Retail customers in the UK mostly use Faster Payments (FPS) for interbank transfers and they clear within a few seconds.