This solution works on both i9300 and i9305 models. After upgrading your phone to Android 4.3 Jelly Bean (especially to i9300, which has 1GB Ram) you may have noticed high ram usage and not fast screen response.
The following steps may reduce even bugs and lags.
Settings --> Developer Options --> Limit Background processes --> 4 processes at most
If these options are locked there's the option to enable them from the top side of the developer options menu. The multitasking won't stop operating after the 4 apps. Apps after the first 4 will hibernate and you will have the ability to re-open them by holding the home menu button for 3 secs. After an app has been hibernated you may see it loading again if you open it by holding the home button for three secs, but most of them will continue operating from the last moment you let them opened.
NOTE: If there's no thing like "Developer Options" on the settings menu, you're not the administrator of your phone. To become an administrator go to Settings --> About device --> Build Number. Touch the build number option (even it is locked) 5-6 times. After becoming an administrator, follow the steps above "NOTE".
