Updated 3/19/12 For v.2.1.1
How to fix slowdown from Mines or Gap Generators
We're using a refraction effect to cause the mine and gap flickering appearance. You can turn down your shader settings in your options if your video hardware is having trouble rendering these extra special effects. This will also impact Phase Tanks.
How to display framerate and network use
In the launcher, select the option to show FPS. Alternatively, while in-game, press ~ or F8 to open the console, then use the command FPS
How to fix blurry text
Under the launcher's "Misc Options" you will see a checkbox for the screen UV bias; switch it and it may fix your problem. You may also want to adjust your resolution.
How to increase performance overall
First, the obvious. Set your details to low, turn off fancy effects, reduce shadow details, drop your resolution, reduce the texture detail. Keep the FPS display on, as it does not harm your performance but will give you a framerate and network rate display that's pretty handy.
Turning off shadow effects is a good way to gain some performance while not making things look altogether worse. I would usually start with that. Turn off high quality shadows (do this if you have an ancient card that shows rainbow shadows), shadows on terrain, and reduce the shadow detail overall.
Turning down the particle and surface effects can help if you generally get slowdowns when a Hind is shooting you; things like bullets, explosions, and impact effects are particles or surface effects. The only downside is you might not notice you are being shot at as much, as surface effects control ricochets.
Some players experience better performance depending on whether or not the game is fullscreen or windowed, and obviously adjusting the resolution might help you out.
If you tend to experience lag while using the Phase Tank or near a Gap Generator, be sure to disable shaders. Those are the primary fancy-pants use for them. Note that in-game, these options are found under "Extended Options."
Registry Tweaks for Even More Speed for Advanced Users
After you have tried every other change, if it is still going too slow, there are a few final adjustments and verifications you can do, but it requires editing the registry. This is a chance to screw up your system pretty severely, so don't do it if you're not comfortable with registry editing.
Editing the Registry requires admin privileges and can screw your computer up so don't do this unless it is your computer and you feel comfortable with it.
It's honestly not that difficult to make these changes, but if you accidentally delete registry keys, your system might not work anymore. So please only do this if you have no other choices. I won't feel or be responsible if you break your own computer.
Open your registry editor (regedit) and locate your Software/BlueHell/APBSetup folder; this is where configuration data is kept for Gamma. I am not going to explain in detail where to find or edit this stuff because if you don't already know, stop now and close the registry editor because you will probably ruin your computer.
Once you're there, you will see all sorts of scary settings.
If you have not already done so safely by using the Launcher, you can now set your Shader Detail to 0, and your High Quality Shadows to 0. If your framerate is utter crap, there is probably no point to have VSYNC on either, so 0 for that.
Under /Render/ you will see most of the settings the Launcher already set up. You probably should never touch anything here even if it looks tempting.
Under /System Settings/ you will find most of the things you want to adjust. There are a few things you should adjust down here if you really are desperate for more frames per second. By adjusting the LOD budget we can try to render less.
Look for Dynamic_LOD_Budget and Static_LOD_Budget. At the lowest settings, they will be at something like 10000 (500000 at highest). You can set these two Budgets to 4000. What you are doing here is forcing the game to start taking a more proactive effort at culling things you don't need to see. Some objects require more thought to display. What we are doing here is helping the game decide to drop Level of Detail for everything it can.
Dropping Level of Detail more often can have the effect of less polygons to render, and less total draw calls. So we are lowering the number of polygons we want the game to think about, to encourage it to stop making draw calls (since the polygons themselves are generally fine and not the issue... your graphics card can probably keep up with 10000).
Also verify that Particle_Detail, Shadow_Mode, Static_Projectors, Surface_Effect_Detail, Prelit_Mode, Texture_Filter_mode, and Texture_Resolution are all set to 0. Most of these should already be at 0 because you have already changed them either in-game or with the Launcher.
Note: If you ever edit your settings in-game, your tweaks to the registry directly will probably be forgotten or overriden. Also, do not touch things like NPatches.