Usually, when your iPhone or iPad is running slowly or stuck, most people mistakenly believe that it is because there are so many apps running which make iPhone or iPad run slow. Actually, this is not true, according to AppleInsider
Apple's iOS platform does allow for intelligent multitasking, meaning some apps will operate or finish a task in the background, then automatically close.
However, the iOS app switcher shows all apps that have been opened on an iPhone or iPad, regardless of whether or not they are actually running in the background.
Misconceptions about the iOS app switcher have led many users to adopt the habit of double pressing the home button and swiping up on all
apps, constantly, in a futile effort to improve their handset's battery life or performance.
For proof, restart your phone, then double-press the home button without launching anything. You'll see all of the same recent apps in the app switcher, despite the fact that iOS has been completely rebooted and nothing has been opened.
The only time you should force an app to close is if it becomes unresponsive or unstable. For example, if you're using Facebook and the app has crashed, return to the home screen, then double press the home button, then swipe up on the Facebook window to force it to close. Reopening Facebook should return the app to its normal state.
If you're truly worried about battery life and performance, actual effective ways of improving it include lowering your screen brightness, manually turning on the iOS 9 Low Power Mode, or digging into settings to disable Background App Refresh (under Settings, then General).
So next time, when your iPhone or iPad run slower, don't force to closing those running apps. You have to check other reasons to improve your iDevice performance.