While you're in the intro/fly-by, hit the tilde(`) key to activate the console. Type in
timedemo 1
and press the enter key. Hit tilde(`) again to hide the console, wait for several passes(5?) of the fly-by and press it again. It will tell you the average framerates for each run of the fly-by.
Note: it is very important that you hide the console because if you don't it will seriously affect the accuracy of the benchmark.