A little more than a week ago, Apple had admitted that it was intentionally slowing down older iPhone models. According to the tech giant, it was doing so in order to prevent battery problems (and potentially worse issues later on). By slowing down earlier models, the company was basically keeping them from shutting down randomly, especially when they are not sufficiently charged, or used in lower temperatures.
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https://www.wirefly.com/apple-issues-apology-slowing-down-old-iphones