Yes, small and explicitly tagged functions can inline directly into the caller. But even when that doesn't occur, Julia is able to avoid dynamic dispatch overhead by doing the dispatch at compile-time, making the function call point directly at the specialized method.

In order to do that with CLOS, we have to add an explicit "inline" directive (as shown by lispm in another comment), because methods can be redefined anytime. Taking a shortcut to an effective method could break the code.