On multiplayer, when the nether is loading any chunks it will lag to almost an unplayable level. Mine-carts can clip through walls and the floor while this is happening. This does not just effect me as everyone seems to complain about it on the server.
Environment
operating system: Windows 7 Java Version: 1.7.0_09 Amd A8 processor with 6 gigs of ram
Nether has always been laggy due to it's lighting.