If you don’t use voxels, I’m sorry but your game will be VERY slow. As eXi said, even using voxels (which should speed up game a hundred times faster) is slow. Imagine a 16 x 16 x 256 world. (A single chunk in minecraft) that is 6400 Actors! Not even static meshes! And that’s just a single chunk, when minecraft often has hundreds at once!
Use voxels, my friend. They are your bet! 
(Oh look! A voxel plugin!)