Some app will not close properly when you press back button in android devices, it will run in background and slow down your device, but you can easily fix this issue by changing some setting in your device, just follow these steps to fix app not close properly.
1. Open phone “setting” tap on “Developer options” if the developer option not available; than tap on “about phone” and tap 3 times on “build number” you will get developer options
2. Tap on “Developer options” scroll down find the apps, check on “Don’t keep activities” this will close app as soon as you leave it.
3. You can also set background process limit, tap on that check on “No Background processes”