ACH transfers are basically instant in the US. But because they can, the sending bank holds on to the outgoing transfer for as long as they can get away with an

ACH transfers are basically instant in the US. But because they can, the sending bank holds on to the outgoing transfer for as long as they can get away with and the receiving bank waits to make the transfer available to the customer for as long as they can get away with. this allows banks to squeeze a little bit more return out of the money, and more importantly, the longer money is in limbo, the more likely customers are to overdraft thus incurring penalty fees. additionally, slow transfers helps keep customer money in same-bank services. if I can instantly transfer money in from my brokerage service, then I have less incentive to use the brokerage service offered by my bank.