The fact that there is an arbitrary money amount shows it's more of a fraud cutoff than a real limit.
In the mean time the credit on your account is debited a 'float' account that accumulates any debt to other banks or third party payment system. Each fast payment system also typically have a reserved amount deposited as a guarantee in the event that a bank runs out of money.
It's a real cost to reserve money so there is a restriction on the sums.
Can the central banks speed up the settlement? Yes probably, but it's a lot of work to do.