ACH clearing (transaction information) happens multiple times per day. Originating bank sends a list of transactions to the clearing house, Receiving bank is notified of transaction pending.

ACH settlement (bank-bank funds transfer) happens overnight. Funds from the Originating bank are "rehomed" to the Federal Reserve, and then again to the Receiving bank. This is, obviously, a database update.

ACH posting (funds available to customer) happens whenever the Receiving bank feels it's appropriate. Generally, by local close of business on the settlement lot schedule (same day or next day). The receiver may choose to delay posting to customer account if they don't trust the Originator, or the customer, or just for fun.

ACH transactions are reversible. This is the reason posting happens as quickly as it does (!), but also the reason posting takes as long as it does.