Ideally, when it comes to performance, you'd want to render a single cube map for all reflective objects in an area, but you'd have to drop dynamic objects, which is perhaps kind of purpose defeating, especially if you're in space with little but stars to reflect. Perhaps it'd be possible to update the cube map generation less frequently so that it spend 6 frames rendering it, then 6 or even 12 frames keeping the previous map, effectively cutting the strain in half or more? It may not look as nice, but it's much more viable for those with less powerful machines.