I really have no idea, I know that mipmaps will increase the framerate, but I don't know how to make and use them.
Here are models info:
Tree:

Grass:

BTW, I didn't use sprites for grass.

I don't know but when i used the USE_BOX flag, my game was slow. Maybe that is the problem.
xxxxxxx

???

a) for models in MED: Skin Settings -> Texture format -> Flag 'Mipmaps'.
b) or use DDS skin with included mipmaps.
c) or at runtime: bmap_to_mipmap(bmap_for_entity(my,0));
d) for sprites: mip_sprites = 1;

Also see 'd3d_mipmapping', 'mip_levels', 'mip_flat' and 'mip_shaded' in the manual.

Thank you, Harry Potter, this will be definitely useful to increase the framerate.
xxxxxxx, yes USE_BOX is slow, but there is no way for me to make smooth movement without it.

Still, problem exist

Again, it is the alphablending. Your grass has an alphachannel and it does not write into the depthbuffer, which means that every pixel is rendered for every grass plane every frame. And those are blended together, which means that the previous colour has to be read and mixed with the new one for every grass plane. This causes your bad framerate if many pixels are covered by your grass. This is a general problem, which will be the same with any engine. You can improve the performance with alphatesting for the completely translucent parts, but I doubt that it will help that much. The only real solution will be not to use transparency and instead do alphatesting and depthwriting or less grass.

In EVERY of my projects since a4 i never managed to have a good framerate (>30). Sure in minigames, top down shooters and whatever it was ok. But i never managed to make a simple 3d shooter with a big map and some ais with a good framerate. With good framerate i mean over 30. Yes i know, u will say its my fault. But i hear a lot of people spoke about the same prob.

Edit: I think this engine just isnt good for 3d ego shooters. Ok, games like left4dead even run with about 30fps. But they have a gigantic map, lot of shaders, lot of pp shaders and hunderts of attacking enemys plus 3 friend-ais around u. u cant compare that.

Although I never made a shooter, I did make a merchant simulation back in 2008. It ran great, had multiple towns with dynamic buildings, vegetation and a AI. It ran smooth on my 100€ hardware (ATI HD2400pro, AMD Athlon 64 2Ghz, 1 Gb RAM). And with smooth I mean 40-50 FPS.

So yeah, it is possible to have good framerate, its certainly not easy, but possible.

Making a today styled 3d ego shooter with pp effects shaders and hell knows in 3dgs = simple not possible, my opinion.

Some guys could tell take a look at Supercan... I wont cause Supecan is a professional game, I'm not a professional...
Plus it's written in LUA, I don't know it.. it's may be faster than lite-c, I don't even know.

They (Valve) had also to think about performance. You just can't have a big map the whole time in memory and let it render every frame, you have to stream the map. Having thousand of functions running every frame with their own stack is also a no go for performance.

The straightforward way of just pushing everything in the level and then blaming the engine if it fails might look easy and like a way worth going, but reality is that you have to think too. Sorry if this destroys your view of the world now.