I found this in another forum, I don't know if there might be an easier way to do it........
JediKnight
Start game
go into multiplayer
hit <shift> + <~>
type devmap ffa_bespin, hit <enter>
after demo loads, hit <shift> + <~> again
type timedemo 1 <enter>
type disconnect <enter>
hit <shift> + <~>
type demo jk2ffa <enter>
after demo finishes, hit <shift> + <~>
page up until you see the frames per second score