FPS limiting is setup for shitty graphics card drivers to prevent them from overheating and chewing themselves up. This process eats a shit load of CPU cycles, so it's no wonder you saw an improvement. For 95% of users, you should never have to use that option. Vsync is similar, except it uses the GPU, and its driver driven, rather than application. And you shouldn't have to use this unless you're pulling 120fps on a 60hz monitor, and experiencing tearing (screen will appear to flicker or "stutter" at high fps). Honestly I have no idea why this option is turned on by default.
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