Forum Thread: How to Make Your Samsung Galaxy S3 System Work Faster

The simplest way is to factory reset your phone. Another way is to install a custom rom like cyanogen or revolution hd.

I'm still using my Galaxy S3 I9300, over the time my phone became very slow, so slow that I almost gave up and bought a new phone, but what solved my problem was rooting the phone and replacing the ROM with KitKat 4.4.4, now it's super fast and I don't need to buy a new one.

First you need to root your phone, I used the free eBook from this site: https://galaxys3booster.wordpress.com, but you can also search Google, some of the rooting tools are free and some cost about $30. After you rooted your phone you can replace your ROM, you can find guides in Google, I used the guide from the site above.