I've just got an idea, but I'm really busy and don't have time to test it.
As I mentioned the performance hit most likely comes from DXT3/5 textures which support semi-transparency.
However my shaders use dithering instead of transparency to render trees. This dithering is still based on alpha values from textures, so without alpha trees will look ugly, but I can probably do something about it.
Now, what I'd like to test.
What if DXT3/5 textures for trees are changed to DXT1 with on/off alpha or even solid ones?
Would it improve the performance?
I don't have time to test it, but it is not too difficult if you know what you are doing
You need to find **your WOFF folder**\WOFFScenery\Textures\Sum Spring Tex\ sub-folder. There are 10 textures like Deciduous*.dds and 4 textures like Coniferous*.dds. These are DXT3.
Make a backup copy of them and store it somewhere outside WOFF folder.
Then use your favorite image editor which supports DDS and resave them as DXT1 (preferably with mipmaps). Or, you can just take any solid DXT1 texture 256x256 (or any size) and use it instead of these existing textures. It can be just solid black, we are not testing image quality
Then test it on a tree-heavy airfield. Obviously you must select spring or summer season for these updated textures to show up.
If it indeed improves the performance I can probably update my shaders to keep using the alpha for these textures, while CFS3 engine will be tricked to believe that they are solid and thus will use faster rendering.