Minecraft wiki entry for Time-saving_tips, associated text: Lag may arise from a variety of causes, depending on your computer. Possibilities include having too many mobs or items in one place, complex redstone circuits, large forest fires and so on. If the game is normally fast enough for you, but occasionally suffers from lag, don't forget the option of temporarily reducing the game's video settings. Change the render distance to 'Tiny', set particle effects to 'Minimal' and turn off any other video settings that you can live without. When the problem causing lag has cleared, go back to your normal settings. You can also press F3 + A to reset the chunks. This can help reset the lag.