Minecraft, the hit sandbox game from Mojang, is notorious for how poorly it runs. Optifine is an easily installable (seriously, you download it and run it. It automatically installs itself into your game) modification for the game that makes it runs much smoother on any computer. Optifine allows you to render higher quality graphics, from doubling frames per second to rendering more chunks to a hi-res texture pack. Optifine also has a couple of useful features like a zoom function and added options under video settings. Once you've used Optifine, it becomes painfully obvious how poorly Minecraft runs without it.
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