As we all know Apple it also updates really old devices, which is confirmed by the recent update of iPhone 5S and 6 to iOS 12.4.8. The speed test of this system was conducted by channel iAppleBytes and you can check it out under the paragraph. It's nice to see how Apple it also updates these devices. However, it is necessary to add that none of the iPhones really abound in speed. However, as long as it serves, why change it? Do any of you still own an iPhone 5S or 6?
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Considering the speed, I don't know if such updates are really "good". If I had such a phone, I would prefer to have, for example, ios9, which was much faster.