I love self-checkout. It is always faster than using a cashier, for the following reasons:

(a) I already know what I'm buying and have a mental map of how to bag it. No sorting or anything required.

(b) I am motivated to get through in a way that cashiers are not. Customers self-sort into shorter lines so cashiers who move quickly just get more customers. No cashier who gets to go home at a set hour is going to move with as much purpose as me (I get to go home as soon as I pay).

(c) Usually there are multiple self-checkout scanners per line. As a result, the line can only be slowed, not stopped, by one person taking forever.