I was poking around the nether in creative mode searching for a fortress when I came to a dead end. I decided to dig up to try and go over it, but ended up hitting the "surface" bedrock. I discovered something strange up here, which may be causing some of the nether lag, a bunch of mushrooms were spawned up here. The attached image is a modded client to allow a minimap, but I have also been able to produce this on a non modded client. I think this, and part of the generation for the nether, is causing some of the tremendous lag the nether presents. If you need more shots, I can produce more for you and from various people as well.