Theres too many opinions in the chat. From research, this is the closest answer I have found over the years.

Yes, iPhones just like all other phones get slower over time. This is because hardware parts get used up over time such as RAM, SSD storage. But this aging is a bit slower compared to other things.

Two of the bigger culprits for slower phones is Software and Battery.

Battery: As you battery degrades and once it gets down to 80%, the phone is programmed by software to throttle the CPU thereby making the phone a bit slower to decrease load on battery and increase longevity. You can bypass this by getting a battery replacement.

Software: As newer softwares come out, they increase the load the phone is under in ways we don’t directly see. Newer softwares although made to be compatible with all phones that support it is still made at the end of the way after the thought of the newest phone. Since newer phones have much more powerful chipsets, they handle the background tasks and software better than older phones. Hence why if you get an iPhone 14 now, in 6-7 years, it may not be as slick and fast but it will still maintain somewhat of its original performance as best as possible. Will you be able to tell the difference? Not unless you compare your iPhone 14 to an iPhone 20 in 6 years running iOS 22