Unfortunately, the devices slow down after some time of usage, especially when you go through installing a lot of apps, basically the same problem you get with computers.
These problems can be solved very easily though. You can follow the tips in this post and try to restore your smartphone back to it's original state without having to do a factory reset all of the time.
1. Keep your internal storage cleanTry and not to clutter your internal memory with a lot of files, if you have free space on your external storage (sdcard), try to store them there and keep your internal storage with some decent free space.
Clean your cache from time to time aswell as that also begins to take up some space especially if one or two months pass. Check this post.
2. Run checks with anti-virus softwareEven though the Play Store looks like the safest place to download apps, there are times where people upload malicious apps which fill your device with adware and other kinds of things.
There are many anti-virus software apps available in the Play Store. One of them is Avast! which is completely free.
3. Beware of apps with adwareMany good looking games and apps unfortunately happen to install adware to your device which gives you annoying homescreen shortcuts aswell as ads filling up in your notification screen all of the time, it is advisable to get rid of such apps once you notice you are having such problems.
4. Uninstall unnecessary appsIf there was an app that you've downloaded only for a temporary use and you don't need it anymore, remove it. There is no need to keep it as it may even run in the background.
5. Update your softwareWhen manufacturers release new software for your phone, it's for a good reason and that is performance improvements and many more. Always check if there is a new update available for your device.
6. Search the Play Store for useful appsThere are many useful apps that can also help you improve the speed of your device. Apps such as startup managers, task killers can be of great help.
- Advanced Task Killer - This app kills all of the unnecessary apps running in the background with just one tap and the best part is, you can kill the app itself so it also won't run in the background.
- Startup Cleaner - This app allows you to choose which apps you want to start up with the device and which apps you don't want to.
7. Restart your deviceRestarting the device sometimes also helps whenever it becomes dead slow but this is just a temporary fix.
8. Turn off animationsThis is quite an effective way to speed up your device. Low end devices have trouble performing these animations and the results are you going through a very slow transition animation everytime you go to a menu or go back.
Usually the settings for that should be in: Settings -> Display - Animations.
9. Remove live wallpaperInteractive wallpapers look sweet, but at a price. The live wallpapers take up quite some resources especially for low-end devices. It is recommended that you stick with a normal wallpaper if you wish to have better performance.
10. Factory Reset (Last resort)Your last resort would be to use the Factory Reset option which will wipe everything and should bring your device back to it's original state as it was when it was new.
Be sure to make a backup of your important files if you are going to do this.