Fixing the texture size will likely double the FPS (if not more) for most users in the test level. The other issue is in the shader code itself. Doom 3's shaders are well written and highly optimized, however the one in the example level is not so much so.

There are other issues that can be fixed from within the engine itself (an initial rendering pass without textures to remove faces that will not be visible - and thus reduce the overall calculations needed once a shader is applied), but those are really pretty minor when compared to the shader itself and the textures being used.