If you are having framerate (FPS) problems than visit our Framerate and Visual Tweaks page which details how to get the best out of Doom 3. 

You should always ignore the first benchmark result if you are not appending 'usecache' to the timedemo command. Doom3 often pauses while loading models and textures for the first time.

1) Load Doom3 using the shortcut that enables the console
2) Open the console using the ' key below ESC or press CTRL+ALT+~ (US Keyboards) or CTRL+ALT+' (UK Keyboards)
3) Enter the command timedemo demo1 usecache
4) Wait until demo has finished and note your FPS

Repeat with the other Doom 3 benchmark

benchmark

To test your FPS while online type "com_showfps 1" at the console

See the FAQ How do I access the console? for details on how to enable the console.