Extreme Performance Issues

Minecraft gets EXTREMELY laggy after it has finished loading regardless of render distance or in fact any setting. This even happens on tiny render distance. I have had lag before but not to this extent. I don't know what's causing this but I really hope it gets fixed otherwise Minecraft would be unplayable.

Where lag usually doesn't happen:

• Superflat worlds
• Basic Terrain
• The End

PS. Optifine/updating LWJGL doesn't make a difference whatsoever, in fact it actually slows it down even more.