There are two issues with older phones running more slowly, and you are probably encountering both of them 1. Newer versions of iOS demand more processing powe

There are two issues with older phones running more slowly, and you are probably encountering both of them 1. Newer versions of iOS demand more processing power because they do more (or perhaps do it less efficiently). 2. Old batteries can't deliver as much power. So an old phone will new version of the OS mores slowly than a new one, regardless of the battery situation. But, if the battery was marginal but hadn't hit the limit before the upgrade, then you get a double whammy 1. The phone simply can't run a more demanding OS as fast as the old one 2. The increased demand pushes the battery over the limit At some point, the battery was going to age out anyway, so it was only a matter of time until you hit the second issue. But the upgrade made you hit it sonner rather than later.