A lot of issues can cause your phone to run slower than normal, but normally it's apps running in the background. Firstly, check settings/apps/running apps first, and see if there are any unusual apps running. If there's anything that you haven't used recently then force close it, or if there are any that you don't use uninstall them. Also sift through your apps and uninstall any you don't need or use; a lot of apps have background services that cause nasty side effects or use a huge amount of resources. The phone has 1Gb RAM so it should be able to run quite a few apps, but with it being a Samsung a lot of RAM will be taken up by TouchWiz (the user UI) so clean up your homescreen of unnecessary widgets. You could also try an app called Greenify: it hibernates apps so that they use less RAM/battery, whereas app killers/RAM optimisers close apps, some of which then try to reopen causing a cycle that burns your battery and uses even more RAM. Also try cleaning your app caches (you don't really need an app for that: reboot your phone) and see if it makes a difference.
Like I said, there can be several causes and this is the simplest answer. You could root your phone, remove the stock launcher and use a third party launcher which would certainly speed it up, or even flash a custom rom, but these are pretty extreme things to do just because your phone is lagging.