See the response to my comment below. My impression is that the best solution is actually a high-thoroughput cryptocurrency for settlement.
But cryptocurrencies are philosophically opposed to central control. And authorities are generally behind the times when it comes to technology. But it boils down to outdated technology based on "if it ain't broke, don't fix it."
I know the Fed is responsible for resolving transactions between banks, but one would expect they'd have a very very broad and resilient system.