FPSLock

Misc > FPSLock gives you more options to set an FPS limit, unlike Vanilla.

.fpslock fps <num> - FPS limit to set when the world is loaded. Set to 0 to disable.

.fpslock menu-fps <num> - FPS when the GUI is opened. Set to 0 to disable.

.fpslock no-focus-fps <num> - FPS when the game window doesn't have focus. Default: 1, 0 to disable.

It is recommended you turn off VSync in options and use the FPS Limit option when possible.

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Usually you will see people (or even yourself ! ) running Minecraft at 200+ FPS. This game simply doesn't need it. By setting an FPS limit you can make your PC run quieter, cooler or run many clients side-by-side. For example AFK fishing! Because you don't need high FPS when you're not even playing the game yourself ;)

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Why VSync off? In short: just disable it and set FPS Limit slightly higher than your monitor's refresh rate for smooth gameplay. 60 Hz monitor -> 80-120 FPS lock.

VSync is there to combat screen tearing - where one half of your monitor shows an old and another half the new image, tearing the real picture apart. Unfortunately it noticably increases input lag and screen tearing is not always apparent or an issue at all.