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

Ever since I got my 14 Pro, I stopped using it altogether. Battery is amazing and I hate that it turns off 120hz variable refresh rate.

Also I have used it in older phones too and in my honest opinion, it never made a drastic difference. It felt like placebo

As typical fashion, to me it feels redundant and gimmicky. whatever you can do with it, you can do just as well and faster with other menus and gestures. clicking it brings you to the app which you can also swipe ur home bar for without having to reach so far up. holding it opens a small menu which again ppl that are used to doing with control center or from the app itself will do faster. dynamic island esthetically is pleasing but lacking in practice