This is post I made for my squad almost a year ago. I figured I'd share it here in case it helps anyone tweak their game settings for a better experience. My understanding of these is simply based on experimenting and taking a good hard look at the world on different settings. I used the in game FPS counter to gauge the performance hit from each one. If anyone disagrees with my assessment on these settings and has other information, please offer it up.
Go to Single Player, Quick Mission, Free Flight France or England. Just buzz around there to test settings out.
Leave this on high. It affects how many polys are used in drawing aircraft, and the distance it begins to draw them at. It's critical to spotting and IDing planes during gameplay.
This controls how much of the rendered urban buildings get final trim pieces like chimney stacks. It has no effect on smaller town buildings. Each of the 5 settings offer no more than a couple FPS difference under the worst case scenarios full of urban buildings.
This controls how many polys are used in the terrain geometry. Since we are dealing with extremely flat topography with the Channel area, it makes little difference, and yielded no FPS gain by limiting it. I have it on high, just to make sure the landscape appears accurately.
This is one of the more consequential settings. There's a good FPS increase with forest off, and smaller gains between each setting with some trees. This controls all trees though, not just forest patches. So turning it off will leave you without any trees at all, making the landscape just look wrong. The other settings scale the amount of landscape trees and the density of forests. There are still plenty of scattered trees on very low, but the forests are very thin. To my eye, forests look unnaturally thin until high, so I use that, saving a few FPS from very high. But this is one to play with if you need to scrounge up FPS.
This controls the quality of tracers, smoke, fire, fuel spray, dust, etc. Unless you get overwhelming amounts of these on screen at a time, you can get away with these on high. Low actually turns smoke and fire off, which is unacceptable to me. Medium uses their low res versions, which still look okay, and offers some extra FPS in exchange. This would be a reasonable compromise except that on medium, the tracers still look terrible. Fuzzy blobs instead of bright streaks. I have to recommend the high setting for that reason.
This only controls the resolution of the plane's skin. Typically you're just seeing the tops of your wings from the cockpit. They look good on Original, and without any performance difference, there's no reason to lower this.
This helps to smooth out the hard pixelated edges on objects. While the list goes to 8, the game only supports 2, so put it on 2. It's better than nothing, and doesn't seem to hurt FPS at all.
Turn this off.
This is an extra lighting process, which is supposed to be more accurate. To my eye, things just look a little different rather than noticeably better. It is worth some FPS, so I'd turn this off.
Turn this on to prevent screen tearing.
This is the quality of bullet holes and craters. No FPS impact, so use high.
This controls how many buildings get rendered in their full geometry and textures vs a simple monochrome rectangular block. Unlimited still draws the farthest ones as big gray blocks. But it renders many more buildings in full geometry and textures than in the other three settings. There is a huge performance cost for this, so don't use unlimited, no matter what card you have. There are smaller, but not insignificant differences between high/medium/low. If you notice frame rates getting choppy on high over urban areas, medium or low on the Buildings Amount will help.
This is a big one. This controls the texture quality of the landscape, so it has a huge impact on game appearance and performance. The landscape draws in squares of degrading resolution dependent on proximity to the player. On high, you get the highest res landscape and the rate of degradation is at a maximum distance. On medium, you still get the highest res landscape close to you, but it begins to draw lower res versions closer than on high. On low, you get lower res landscape even up close, and it degrades quickly from that.
Also, on high and medium, there is a texture overlay when you are very low or on the ground that helps break up the landscape texture when you are extremely close to it. On high, this is a higher res version than medium. On low, you do not get any close up texture layer to break up the already low res landscape, so it looks really bad on the ground. Depending on your GPU, try to get away with at least medium. It looks a lot better than low. High looks great, if you can stand the FPS you get with it. But if you really need better frame rates, lowering this setting makes quite a difference.
When very low, it loads big swaths of grass, albeit not always at the proper distance. So sometimes it will just pop in around you. This is a nice look and helps a bit with ground appearance, especially when Land Shading is on low, giving it something other than super blurry textures on the ground. Since you only see it on the ground, FPS is a non-issue, so leave grass on.
Another big setting. This controls all shadows in the game, for trees, buildings, planes, canopy framing, etc. While not preferable, the landscape looks okay without them, although it would be nice to see low flying plane shadows. More than anything, they provide a real nice effect as they move across the cockpit during maneuvers, so they can be hard to give up. But this setting offers a big FPS difference, so this is one to consider leaving off if you need a smoother frame rate.
You need to see roads, as they help navigation. No FPS hit, so leave them on.