By default the processor in the Raspberry Pi runs at 700MHz, but it can be overclocked without voiding your warranty. Basically a processor is designed to do one job at time, be it retrieving something from RAM or adding to numbers together, its limited to one task. But when we’re using them the idea of one thing at a time is hard to get our head around since it appears to be doing so much more. That’s because a processor can do that one task really, really, really, fast. The clock speed, 700MHz, give us an idea of how many tasks it can do per second; the higher the speed the better performance you get.
Overclocking simply means increasing the clock speed past its defaults. The problem there is if you overclock to much the processor becomes unstable and can lead to crashes or even burn its self out.
My Raspberry Pi is running Raspbian so to overclock it simple type
Go down to item 7 Overclock and press ENTER, press ENTER a second time to confirm the warning message.
raspi-config has five levels of over clocking: 700MHz (no overclocking), 800MHz (modest), 900MHz (medium), 950MHz (high) and 1000MHz (turbo). All of which are supported by the Raspberry Pi foundation and will not void your warranty, over clocking to anything other than what’s on this list or overvolting the Raspberry Pi will void the warranty.
Select the level of overclocking you want from the list, as bellow, and click on <Ok> to confirm your selection.
After that your Raspberry Pi will need to reboot for the new settings to take effect. After a reboot you can test your settings by looking in /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq
If for any reason your Raspberry Pi fails to boot after you’ve overclocked it hold down the shift key at boot time to temporarily disable overclocking then just go back into
sudo raspi-config and select a lower speed.