i have the same problems too, one is because lava, you get so laggy you can't even move, second is taiga biomes (snow forests, plains) because there is so many blocks (including the snow!) it has to load A LOT more than usual, third is if there is a lot of entities (entities can be mobs to blocks in mined form to falling gravel to primed tnt) it will need to load a heck lot of stuff, and most of all, redstone. yes, it makes the awesome electronics in minecraft, but while the redstone is on and in progress of turning on and off stuff, it lags soooo hard sometimes, it's unbelievable. i hoped i really helped, i feel your pain. >-<