i made a nice, spiky terrain with nemesis terrain generator. i exported it in .rmf format, opened it in hammer 3.4, added some entities (the problem really has nothing to do with entities, i think). i then compiled it with hammer. what i got is @#$%(&. i also found that when i saved the map in .map, all the 'triangles' (what i made with terrain gen) were given the aaa texture, and they were all just... have a look yourself. compare the rmf and the .map.
What's the problem?
Take a look in 3d mode and fly around. You'll notice that the intire map is divided into many small triangles, and the spikes on top of those. The compile has to render each and everyone as a seperate brush, and since they are conncected split them up and the brushes that they split up need to be splitted, then light has to bounce everywhere on the map. It probably doesnt work becuase it's just an insane map. Don't use generators. Period.
Since I am feeling generous, I'll try to make a copy of this map but with realistic properties.
eventually you will have a map that looks sort of like yours, but slightly less visually appealing and it runs. you will also be able to make it 10 times bigger.
open my or your rmf. copy everything. make a new map and paste everything into that map. when you export to a .map file, the crap should be gone. this worked for me. i bet most of your ideas could be made, it's just that there are few mappers who can make their ideas without some loss in appearance/size.
did you make a skybox around the map or make it connected to the bottoms of the ground?