lighting engine issue

in 1.4.7 and all snapshots up to 13w05a, I get 80-90 FPS with smooth lighting on. in all snapshots after 13w05a, my framerate is at 20-25 FPS with smooth lighting on and extremely squirrely. idk how to explain that part. i have found other bug reports similar to this, but noone has ever found the exact reason for the massive framerate drop. i have found that it is an issue with the new lighting engine as whenever i turn smooth lighting off on any of the newer snapshots, my framerate shoots back up to 80-90 FPS.