Hi there! There are a lot of reasons for your phone to slow down.
Some of which are:
1. Not enough memory
If you packed your phone with a tons of apps and data, it can slow down your phone. Consider removing apps that you rarely use to free up some space.
2. Cache needs clean up
There is a cache partition on most Android devices that stores items that are accessed the most. After a while, it can become clogged with junk, causing slow performance. You can usually wipe the cache by accessing the recovery screen at startup. Each device has it’s own method for accessing the screen.
3. App services running in the background at start-up
Some apps run constantly in the background. Restart your Android, then check under Settings > Applications > Running services to see the list. Uninstall the apps associated with the services you don’t need if possible.
4. Your device is too old for the version of Android you’re running
Check updates regularly.
5. Widgets on home screen
Remove unnecessary widgets.
Source: 1, 2