You don’t need to close iPhone apps by double tapping the home key and closing them. This is actually a list of recently used applications. Most of the items on it are not actually running in the background. They’re just stored so you can go back to them quickly. Your iPhone will automatically remove an app you aren’t using if it needs the memory space. If you remove them yourself you are actually slowing your phone down because it will have to completely reload the app when you go to use it. There are some apps that run in the background, but in a very limited way. These apps will continue to run even if you take them of the recently used item list. If you want to control apps that actually run in the background you can do it from Settings.