I've seen people make various suggestions about settings to increase frame rate so I thought I would test some on my system*. My default configuration (which is Post Lighting off, shadows off, reflections on, Environment Map set to None, Tree Detail set at 18%, Detail Range at 3 miles, Object Detail at 100%, 4096 textures, graphics card set for 4XAA, MSAA, 8XAF) gives a baseline frame rate of 37 fps. Here are how some changes to a single parameter affect the frame rate (an average of 8 values, 4 locations and min and max view zoom). Note these locations are all close to or on the ground; frame rates when flying at altitude are usually pegged at 60.
Texture Size: 4096 2048 1024 512
Frames/sec: 37 37 37 37 (yes the textures got progressively worse so it was changing)
Field of View: 80 106
Frames/sec: 37 39
Option: Shadow off Shadow on Reflection off Reflection on Clutter off Clutter on Post Lighting off Post Lighting on / AA off Post Lighting on / AA on
Frames/sec: 37 24 45 37 39 37 37 37 36
Tree Detail: 18% 50% 100%
Frames/sec: 37 26 15
Object Detail: 0% 50% 100%
Frames/sec: 44 43 37
In summary, on my system the settings that significantly reduce frame rate are high Tree Detail and Shadows On. Although not tested here, setting Environment Mapping above None also significantly lowers frame rate. Texture size and FOV had little or no effect on frame rate. Your mileage will undoubtedly vary.
* Intel E8400 @ 3GHz, 8GB RAM of which my 32bit Windows XP only sees 4GB, Radeon HD7870 w 2GB, 1920x1024, DX9