**Edit:** \ It doesn't appear to be the speed at which you change dimensions, but the fact that changing dimensions twice (IE: overworld -> nether -> overworld) appears to break something in the background, preventing DH rendering until Minecraft is rebooted. ~~If you rapidly swap between dimensions (in my testing it was back and forth between the overworld and nether once every 10 seconds or so) there is a chance to break the system, causing nothing to render until Minecraft is restarted.~~ ~~I'm guessing the DH Level loading is interrupted gets stuck in an in-between state.~~