Only the smallest local bank or credit union would not have the ability to transfer money online via ACH for their customers. Pretty much everyone can login, go to the transfers tab, and enter an account/routing number to transfer money to (or give their account/routing number to another bank to transfer money in).

If you do not, then you should probably sign up for a free account at any of the multitude of online banks that do have the capabilities.

For example, at CIT Bank, which I easily opened a free online account, I have ACH limits of $250k outgoing and $500k incoming, PER DAY. All I had to do was enter my account #/routing # for my other bank accounts.

Zelle does not set transfer limits, the banks do. For example:

https://www.bankofamerica.com/online-banking/zelle-transfer-...

Zelle is just a payments network.

https://en.wikipedia.org/wiki/Zelle_(payment_service)

Also, as far as I understand, the mechanism of ACH prevents it from being instantaneous, requiring old school processes every night to reconcile and finalize transactions. ACH does not check if accounts have money in real time, it assumes it, and then deals with problems later, which is why transactions take days to finalize.

Well they allow any other bank/credit union to use it, and based on the fact that 1,600 financial institutions participate/80%+ of the US population, it must not be costly (or or is free).

It sounds like the US government was asleep at the wheel and the big banks got together and did what they had to to enable US residents to be able to instantly send money to each other electronically.