Like some of the other comments said, ACH payments are handled by a middle man(The Clearing House/FedACH). In terms of speed, this is what their current guidance is:
"Specifically, the Nacha Operating Rules require that ACH credits settle in one to two business days and ACH debits settle on the next business day. Recent enhancements to the Nacha Operating Rules now enable same-day settlement of virtually all ACH transactions."
(https://www.nacha.org/news/what-ach-quick-facts-about-automa...)
For what you're asking about with check reversals, there are protections that let you challenge payments after they have "cleared". ACH payments from individuals can be challenged for up to 60 days and for businesses, up to 3 days (although sometimes banks will let you do it beyond that window).
ACH is built to be reversible. If you want avoid that, you can use wires or (soon) RTP.