A little while back I was easing back into programming with making a voxel engine. It worked, but all the physics impostors really hit the fps hard. I tried a method where EVERY mesh in the scene was checked to see if was near the player, then added a physics impostor if so. That also hit the fps hard. My new solution: rays.
A ray is casted down. whatever mesh it hits, has a physics impostor. The hit mesh changed? dispose of the old impostor, and add the new one. Works at around 30fps, and now I just hafta optimize the rendering.