Banks that aren’t direct participants in the Faster Payments Scheme are often far from instant. I used to have a current account with Norwich & Peterborough BS, and could only choose a banking day for a payment. So if setting on up (online) this evening, I’d have to choose tomorrow as the transfet date. Transfers then only happened at the beginning or end of the day (depending when you set it up). As @anon44204028 says, this could lead to multiple days if a long weekend is involved.
I think these banks essentially have to transfer the money to another bank (eg HSBC) and then that bank dies the FPS transfers. I guess smaller banks also have less IT spend, so probably work with older batch-based systems?