Rooting immediately voids your phone's warranty. After they are rooted, most phones cannot be serviced under warranty.
Rooting involves the risk of "bricking" your phone. A bricked phone is no better than carrying around a brick in your pocket.
A bricked phone means one thing: your phone won't turn on in any way, shape or form, and there's nothing you can do to fix it
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If your phone is rooted, download an app called SetCPU you can overclock your phone for better performance, or underclock it for better battery life with it.