Loren told me how.

In the game press the ~ key.
That pops up the console.
Then type: console.showfps 1 and hit enter.
That then puts a framerate counter up in the left hand side of the screen.
You'll have to turn off the radio commands (f8) to really see it.
And it doesnt show up when your somewhere dark, because the numbers are black.
Press ~ again to close the console.
Z~