Preferably, if you want to speed up device performance, you may want to clear up RAM via task manager (Hold down home button and depending on OS version, tap the pie icon or tap "task manager"button, tap the RAM tab, and then press clear memory). If that doesn't help, you may want to think about uninstalling any apps/games you no longer use. Rooting your device will also help in uninstalling any bloatware carrier applications installed within your device that take up significant amounts of space.
Judging by your phone's model number, your handset is the Samsung Galaxy S3, right?