The US ACH system does rely upon COBOL mainframe systems which only support batch processing. But that constraint does not cause the multi-day process. In fact,

The US ACH system does rely upon COBOL mainframe systems which only support batch processing. But that constraint does not cause the multi-day process. In fact, since September 2017, the US ACH system has supported same-day ACH processing (with a slightly higher fee of 5.2 cents). The main constraint has been more related to operations. There are thousands of people whose sole job is to process ACH files. They mostly work at bankers' banks and corporate credit unions (credit unions' banks). But also at financial institutions that range from under $1mm in assets to over $100bn. Most of these jobs could be fully automated away, but they haven't been yet for a variety of reasons.