Many avid iPhone users find it second nature to close recently used apps that are currently running in the background assuming that it will improve CPU and GPU performance and conserve battery consumption, but little did you know that this might actually do the opposite!
Apple's iPhone operating system (iOS) has been specially designed to work more efficiently by optimising frequently used apps, and when you close your apps, you are inevitably force-quitting those applications from the optimisation processes of the operating system which means that when you reopen those apps, the operating system has to restart the entire optimisation process once again which actually takes significantly longer to reinstate.
A good example and way of thinking about this whole concept is that you wouldn't turn your iPhone off every time you are done using it, so a similar train of thought is to simply put your iPhone in standby mode by locking your screen and unlocking it when you need to use it again. So don't feel "electronic sympathy" by feeling that you have to close all your apps that are running in the background each time you have used your iPhone.
Take a look at the video below by the YouTube channel: Apple Explained on Why You Shouldn't Close iPhone Apps...