When I play with my friend in local area network, Minecraft will be very slow. It takes a long time to load the Nether, and the worse thing is when the Nether is loaded, the player will be suffocated in obsidian (I am not pretty sure, but it's very difficult to break. I failed to break it before I die).
It will not happen every time. And only the host player will be suffocated in obsidian, other players will load the nether smoothly.