Apple or as a matter of fact all these manufacturers of electronic devices deliberately slows down your devices after a certain period of time. They do so by installing the updates in the name of security updates.
As a suggestion avoid buying the Apple products as a status symbol as Android versions work better than Apple with more versatility. However, if you have work of high-security nature, then you can consider Apple device as unauthorised apps can not be installed in the device.
Have a look at this case against Apple and you must go through this video, Now it is a choice between devil and death that is whether you want to update the device or not because if you update the device with so-called security patches, you invariably slow down the device.
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