Over time, the phone will have lots of apps and multimedia. Apps running in the background occupy some of the RAM and beyond a point, this will cause a slowdown. One way to get around this is to install a cleaner or speed booster like Clean Master. 

The problem is that you have to remember to use it. With a free app called Lock Booster (by developers Dotc United), you don't have to remember to boost, it happens automatically each time you unlock the screen.

In addition, Lock Booster replaces the default lock screen and gives you over 100 HD wallpapers, real time weather, notifications and app shortcuts on the lock screen. You won't find the app icon in the app drawer so you can adjust settings from the lock screen itself. Options include disabling the booster, wallpaper rotation, screen lock security and notification toggles