Go to "Settings->Game Options". There you will find a number of switches regarding performance. The only settings I turned down are "Particles" and "Dynamic Lights" (but the latter only to fix a problem with my graphics card) and I have a really old/slow PC. The Particles-setting has quite an impact. At one point I think I went through them and checked which ones gave me the best performance but I forgot the results, except what I am using now
.... And yes, changing the resolution does not really help a lot. It's more the amount of objects the game has to deal with, and it does not matter much how big/detailed these objects are.
In "Settings->System Settings" you can also turn down some graphics stuff like "Lighting" which on some maps makes a difference (but also check
http://www.westernquake3.net/viewtopic.php?t=698 about the lighting settings
...)
Edit: That happens when you need more than 10 minutes to write an answer
, now you have the info twice ....