I'm still slowly learning the mod and testing things out for skyblock compatibility. One issue I've found in skyblock test worlds is that the skybox appears to lack a bottom. By that I mean that if you fly downwards rather than the sky continuing below you as it would in GOG or Void World Control you get a flat black square at the bottom representing a plane where bedrock would normally be; the void below beckons in a rather ugly manner.

This should be a very easy fix for you so I figured I'd bring it up. I've left my diagnostic tools and skill bar up in the picture. I'm getting great FPS flying around in the rain. lol