Showing off is a subset of performing; it's a manner of performing. THey're not mutually exclusive - you can't really show off if you're not performing. Steve Goodman frequently showed off shamelessly; I don't know anyone who ever witnessed it who didn't love every note. Taken in the universe of all folks, performing itself is showing off to some extent, vis-a-vis folks who can't perform. I suspect that as it's most often used, "showing off" is a term applied by them what can't to them who can & do.