Running the new 13w05b snapshot, spawning a new world I am suffering from extreme performance loss, running at 30-80 fps with extreme stuttering (very unsmooth). I read about the lighting updates, turned off Smooth Lighting and my fps increased to an extremely smooth 140-200fps. This is confined to the Smooth Lighting setting. I tried Fullscreen to make sure Vsync or anything of the sort was the issue, but the FPS drop was still there

This is a bug, as in previous versions I would loose 5-10 FPS, if that. In fact, running Minecraft 1.4.7 with GLSL Shaders I can run at a locked 55 fps on Normal render distance, and turning off Smooth Lighting on an obviously GPU bound setup gained around 5 fps. This 1.4.7 setup included Optifine for performance and more.

The new lighting is causing a massive, unusual loss in FPS on a relatively high end rig. It also slows down chunk loading slightly, visible but not visible over the already slow loading on an SSD.

Edit: Just noticed this is a duplicate of MC-8824, sorry.