The mainframe batch-processing computers might be efficient. I have no idea. I am taking about the "ACH system". It's supposedly automated but it shuts down during the weekends. There is no way to tell how long it will take for transaction will take to complete. If you listen to the NPR podcast, it took one of their transactions 8 days to complete. And why is "speed" not a measure of efficiency when it comes to transferring something?
We transfer most freight by slow container ships instead of fast airplanes because it's much more efficient: It's less expensive, requires less energy-per-kilo, and amortizes the cost of the vehicle over a larger volume of cargo.
It's a mainframe batch-processing system built in the 1970s, and it's mostly worked just fine for 40 years. That's pretty efficient!