If performance is critical and you don't need strong encryption then there's always rot13. You're probably not going to beat tr/A-Za-z/N-ZA-Mn-za-m/ for speed in pure Perl and it's certainly not strong . . . :)

(Update: Seriously though I'd look at DES or 3DES as you're likely to find pretty peppy implementations and that's probably strong enough to deter casual snoopers.)