The multitasking was created for some purposes, abilities to switch between open apps, and of course ability to kill app. In the past I have done some experiments on my iPhone, at that time running ios 6 to monitor how the iPhone 4S was handling the ram memory with load of apps opened and I realized that the more apps were open in the background the more less free ram the iPhone was left with, while this is good for quick app relaunching, the device was struggling to handle some more heavy apps live iMovie or Keynote likes apps, the struggle was noticeable as the app would take time before giving go to user to interact with, of course iPhone 4S is 500Mb of ram but the apps are built in consideration to that. But when there was nothing in the multitask, iMovie would lunch quicker and work faster.
An app which struggle to free some ram in order to operate properly is likely to drain power. The app that was not in the multitasking and is just lunch will also drain some power, but the things is I don't very often heavy apps to want to have a lot of free ram for the app to run quicker rather than the app first freeing memory before loading completely, so I've decided to not mind about opened apps in the multitasking and have them ready for next use. Also I made sure to disable the background app refresh so they do not just eat up memory, but also keep the CPU busy by refreshing every so often causing my battery to drain more faster. Since then, I was set and things just work.
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