Cubecamera Refraction envMap pixelated

Question

I'm using a cubeCamera to get refraction material on my mesh (here a cube).

var myCubeCamera = new THREE.CubeCamera( 0.1, 70, 512 ) myCubeCamera.renderTarget.mapping = THREE.CubeRefractionMapping;

When a use the CubeReflectionMapping the reflection looks quite good but with CubeRefractionMapping I get a weird pixelated texture like on the picture. I manage to resolve this problem by increase the cubeResolution to 2048 but performance are so bad :/.

Here the fiddlejs

So I'm looking for another way to solve the problem without degraded the frame rate.