In Android, once you launch an application, it will continue to run in the background, even after you’ve left it and moved on to other tasks. After enough applications are opened at once, you may notice your device lagging a bit due to the number of apps eating up your device’s RAM.
Here’s how to kill applications running in the background.
1. Launch the recent applications menu. How you get to this menu will vary a bit from device to device. On the LG G5, for example, you tap the square button to get to recent apps, while on the Galaxy devices the icon looks like two 90 degree-angles.
2. Find the application(s) you want to close on the list by scrolling up from the bottom.
3. Tap and hold on the application and swipe it to the right. This should kill the process from running and free up some RAM. If you want to close everything, press the "Clear All" button if its available to you.
4. Navigate to the Apps tab in settings if your phone’s still running slow. On Samsung phones, this menu is called "Application Manager." On LG phones, go to Settings > General > Apps.
5. Go to the "All apps" tab, scroll to the application(s) that’s running, and click on it.
6. Tap "Force Stop" to kill the process for good.