When it comes to Android mobiles, the major problem is lagging, hanging and battery draining so fast. I appreciate that you have been using this mobile for more than three years. Believe me, I have to exchange every year or two.
Here is the reason why your mobile is hanging and lagging.
1. Your phone or external SD card storage is running out. You need to reserve minimum 500MB to 1GB space in both storages.
2. Installing too many apps. If you have recently installed too many apps from PlayStore, Xender, Third parties websites, the your mobile is more likely to be affected by some firmwares that allow access to restricted permissions.
3. Some mobile devices like Gionee, Asus, and Nokia comes with too many pre-installed apps and games that cannot be Uninstalled. In such cases, if you think, the pre-installed apps are taking too much space then consider rooting your Android device by King Root app. Once you have installed, UNINSTALL unnecessary apps. Here is the list of pre-installed unnecessary apps.
A) Google Paly Music App
C) Google Drive. Please use Chrome to access files instead of drive to save storages. If it's necessary then don't UNINSTALL.
D) Theme Park
E) Pre-installed Games
F) Google Photos (If you already have gallery app, consider uninstalling this app)
G) Background Processor (This app monitors and allow access to apps that can run in background and auto start itself even if you kill them by task manager)
4. Once you root your device, start King root and make sure you check the battery health. If more than 2 blocks are damaged, replace the battery.
5. Install SD Card 2 RAM app. This is a premium version. Try searching online to get it for free. Once installed, create a RAM in your SD card to remove lagging. Alternatively you could try RAM cleaner auto boost to automatically boost junk files and caches.
6. Your phone shuts down because you never updated the firmware and phone software in three years. If you are using the latest version, try escalating the issue to mobile repair person or flush your mobile.
7. If none of the above tricks works, then consider buying a phone that suits your budget. I would recommend Huawei or Redmi since these phones have high configuration, cool designs, and never hangs or lags.
Hackers never learns but always wins!