On a clean install of Minecraft 1.4.7, I have an fps of 120 or more. After I installed minecraftforge 126.96.36.1994 I get a lot of lag. (It's the latest version for MC 1.4.7)
My fps drops below 20 when I start to walk after I wait for the chunks to load. Installing Optifine and tweaking it doesn't work. I tested it without anything else installed. Just Forge.
Specs (As if it matters since everything works perfectly before):
Clean installing Minecraft and Forge doesn't work.
My drivers/programs are up-to-date
The people on minecraftforge.net seem unsopportive when someone says that the game starts to lag after installing forge. While it didn't before forge was installed.
Maybe people in here can help.
TL;DR: Forge causes big fps drop after installing it.