Phones typically begin to slow down as their internal storage fills up.
Try deleting unused apps and photos, or moving them to the cloud or a microSD card. In addition, you should close open apps that you are no longer using, delete app cache, and limit the use of live wallpapers.
An app's cached data can be deleted by going to:
- selecting a certain app
- and choosing the Clear Cache option
You can also check your device settings and ensure your device overall system does not need updating.