Out of curiosity, are there any reasons to prefer slower settlement? (Since this seems like a no-brainer, but clearly hasn’t happened already)
1) the contract
2) the paying of the money
There are ancillary bits as well like unwinding of mistakes.
The problem is this, moving money fast is hard. It can be done, but its far cheaper to settle up at the end of the day, when the total inflow/outflow is calculated.
But crucially there isn't enough liquid capital to service this level of actual transaction.