Irregular ground collision

I was using unity terrain for a level of mine and I just discovered that terrain doesn’t work in iphone. So I decided to create separate meshes each one representing a session of the terrain and load them as the ground for the level. The thing is that the collision detect with the terrain was very good. But with meshes, I’m afraid the collision would be very expensive if I use mesh collider. So, do you guys have any suggestion on how would be the fastest way (if there’re any alternatives, of course) to simulate the collision with the irregular ground mesh?

Maybe it’s a crazy idea, but do you think I could use a terrain heightmap for the collisions when using a mesh(obj/fbx) for the ground?
does heightmap collision work in iphone?