New lighting system severely drains FPS since 1.2. This problem persists in 1.2.5. Lighting changes in-game caused by lava, redstone lamps, redstone repeaters, and/or pistons cause heavy FPS drops and/or stuttering on low- to mid-range computers. This problem didn't exist in any version prior to 1.2. It looks as though there is now a lot of excess load on clients to calculate lighting changes, whereas pre 1.2 Minecraft forced the server to deal with lighting calculations. This essentially makes the nether and busy piston/redstone contraptions unplayable as the FPS lag is extreme.