Performing a factory or hard reset of any device regularly will improve performance and clear up certain bugs that may have been lingering from dodgy apps, previous firmware update hiccups or just prolonged usage and lag. Considering just how many Galaxy S3 owners have said that they're happy to stick with their device in the face of the underwhelming Galaxy S5 and expensive S6, refreshing the Galaxy S3's performance remains timely. Here's how to do it (it also works on the S4).
You have a couple of options for resetting your device. Firstly, you can choose between a basic factory reset, done via the settings menu on the device, and a hard reset, which is done via a hardware button combination and the recovery menu. The latter gives you more options for wiping data, cache and partitions. The other thing you need to consider is whether you want your full system reinstalled after the reset or just your apps and personal data.
Always back up first
Before you even consider doing a factory reset or hard reset you really, really need to back up your system, apps and data to whatever level you need. I always perform an app and data backup on my device with Titanium Backup, then I do a ClockworkMod ROM Manager backup and copy my backups, pictures, music, etc, over to my computer manually with a USB cable, just to be safe.
Keep in mind there's a difference between simply backing up your apps and data and making a complete Nandroid system backup. All of this may seem overly cautious, but after losing all your important data once, you'll soon see multiple backups as a valuable investment of your time and energy.
Factory reset the Galaxy S3
This is the quick and easy reset option and will wipe your data from the device but won't affect your SIM or SD card. No reset will affect your SIM contents, but some people are a little paranoid, regardless, If you're one of them, just take it out first (along with your SD card, if you like).
Then go to the Settings menu, select backup and reset and hit factory data reset, then reset device and delete all. You can achieve the same result using the Android code *2767*3855# from the dialer.
Hard reset the Galaxy S3
This is basically the same as doing a factory reset, but when you use the hardware button combination you have additional options to manually wipe more of your device. Depending on how often you do a reset and wipe or flashing custom ROMs this may be a natural or unnecessarily complicated way of doing it, but I like to be in control, so this is how I do it. You can just do a factory reset and leave it at that, but some people like to do a factory data reset, then wipe the cache partition and then the Dalvik cache separately before a factory reset and then again afterwards, just to be sure. It's all up to personal preference, but if you choose not to do these last two steps and have any issues after rebooting (like getting stuck in a bootloop), wiping the cache partition and Dalvik cache can quite often be all you need to get you back to where you want to be.
The hardware button combination for the Galaxy S3 is to hold Volume Up, Home and Power buttons until the device vibrates or you see the Galaxy screen. Then, release the Power button while continuing to hold the other two. You'll now be brought to the Android recovery screen (you can let go of the buttons now).
Using the Volume keys, scroll down to wipe data/factory reset and confirm with a tap of the Power button. Scroll down to delete all user data and again hit the Power button to confirm. You can now reboot the system from the recovery menu. Once your S3 reboots, you should have much smoother system performance.
It's also a good idea to keep your apps up to date and delete any unused apps to keep things running better too.