The initial subscribe can take as many as 10 days, too. The initial confirmation (opt-in verification) is immediate because that's part of the server where you

The initial subscribe can take as many as 10 days, too. The initial confirmation (opt-in verification) is immediate because that's part of the server where you subscribed, but the distribution (as already explained) happens elsewhere and requires a sync of the updated list (subscribes and unsubscribes) to the distribution service.