Currently I'm using the iPhone 5s on iOS 8.3
I know if I were to update my phone to the latest OS I may notice some slowdown on my phone's performance, since it may not have the "muscle" to handle certain functions.
Many apps now are optimised for iOS 9 and/or the 6th generation phones.
Will my 5s' speed performance slow down if I update my apps? Will the app's performance slow down?
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Will updating my iPhone apps affect my phone speed as how updating the Operating System (OS) will?