20 FPS is bad, but 50 FPS isn't, actually. A constant 30 fps is playable, though most people prefer a steady 60 fps, and this is considered "good" for any game (anymore than that is gravy).
Get the latest stable drivers for your video card & sound card first and the latest stable DirectX for your operating system.
Install the official 1.04 patch (if you haven't already), this may improves things a tiny bit for compatability first off.
Now in setup:
Turn vsync off.
Turn "Shadows" to "simple" (or off).
Turn resolution down to 640x480.
Turn detail to LOW and models to LOW.
Turn depth/textures whatever to 16 bit (instead of 32).
Turn anisotrophic all the way off (all the way to the left I think).
Turn filtering to bilinear instead of trilinear.
Turn sound quality to low and turn off EAX (if it's on).
Turn off Detailed shaders.
Turn off dynamic lights (forget if JK2 has this option or not. If you can't find it, then don't worry).
You can also get a boost by turning on "Simple Items."
There's more but try those first...
If you wish you can also turn off the gun model, force all skins to look the same, and some other stuff, but that starts to make it difficult to play properly.