UNITY Performance Optimization

We want to create an ultralight plane simulation. The flight will limited to a squarred area of 15x15km on a valley (no moutains, no cities). The plane will stay at the same hieght ~ 150m. At present, we import heighfeild and color from geodatabase. We create 15x15 terrain with 2048x2048 color texture and 1024x1024heigh texture. Of course it is slow. We are looking for a report of what is possible to do. Main target is WebGL (GPU of 1G). Our objective - A script that can show/unshow terrain depending on camera (frustrum culling) - A method to automatically add landscape (trees, details...) Our problem, when the ground is flat, the camera looks at the ground with an angle of 30° and trees look ridiculous when seen on the texture. They looks fake. -Any other Idea, maybe it is simpler to choose a upper height for the planes (300m) and use a normal/bump map to make the landscape real. -If you think that our idea of 15x15 terrains is not the best one... You can take a texture on gmap at higher resolution to test. The height maps has no importance, you can draw it by hands. This problem focus on engeneering and algorithm. Any coutry welcome