Nether terrain takes a long time to build and you have been killed before it has finished.

Built a portal and stepped in. Had the Building Terrain screen, which took an age, but before I could do anything, because the terrain building screen was still visible and the green line was still moving towards completion, I could hear the ghosts and them bombing me. When the nether terrain had finally finished building I was already in lava and dead. The second time the same thing happened but I had managed to run a few steps before I finally died. The lag makes the game almost unplayable now. I was not holding any maps!