Lets say you have a big, hollow square in-game (100x100x100 blocks) and you break one block from the top, and then fill it back. Depending on how much light gets updated, you will experience extreme lag. If it updates enough much, for me, it can take several minutes. FPS drops to 0 temporarily, and when it has done updating all that light, FPS will be normal. This didn't happen before, it is something with 1.9.