Old phones certainly slow down over the period of time when they run their course, though Apple phones, thanks to the asking price remained an exception. However, it looks like it is slowly changing as users on Reddit have found evidence that proves Apple appears to be intentionally slowing down old iPhones that have low capacity batteries.
iPhones becoming slow due to OS update was a special case, but now it looks like the company is throttling processor speeds when a battery capacity runs its course or gets weak. This issue was introduced post-iOS 10.2.1 and 11.2.0 updates. iOS 10.2.1 update is particularly relevant since it was designed to reduce random shutdown issues for the iPhone 6 and iPhone 6S and this fix seems to be throttling the CPU to prevent the phone from randomly shutting down.
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Similarly, the Geekbench’s detailed test report states that the iOS 11.2.0 update brought in similar throttling issues on the low-capacity iPhone 7. However, this completely turned to normal once the batteries have been replaced, say Reddit users. Though Apple might be slowing down iPhones to preserve battery lives, the reality is that it will only push users to upgrade the phone, instead of replacing the battery which is misleading.
Apple is yet to comment on the new reports, but since many are reporting that replacing batteries increased their benchmark score as well as the performance of the phone, it will be interesting to see what Apple has to say about this.