I think the speed with which refunds show up is entirely down to how quickly the retailer submits them, but I could be wrong.
I think the delayed from date is when the refund was ‘issued’, but but Monzo doesn’t actually receive that information until later when they get a presentments file (they don’t get a real-time message about it like they do with debits).
…
Just found the source of my vague information. The great RichardR explained the vagaries of refunds here (seems there are a few different ways they can happen):