This has not been my experience. I can usually tell when my phone bogs down loading an intense website or switching between 3 apps after a couple years. If I want my phone to last a long time and I want to enjoy using it I've found that it's best to start with a device that is as powerful as possible.
That's usually a combination of planned obsolesence and bad software, Smasung flagships for example start to significantly slow down after roughly a year of usage. These CPUs are more than enough for daily tasks like web browsing and as soon as you have a manufacturer with good software support or load something like LineageOS on the device it usually works very well again, in some cases you also need a new battery because many clock down once that looses capacity.
We're gonna have to agree to disagree then. I don't play games and currently use a Pixel 6 Pro. I occasionally feel this phone bog down and I am not a particularly heavy user. If I'm loading an intense website while streaming music and then I go to check my email I can see a few frames dropped from its 120Hz refresh rate. If I experience this with a flagship a couple of years old, I can't image the Fairphone would fare any better
Do you use the default Android install or aomething like LineageOS, Callyx or /e/OS? I use a Oneplus 6 with LineageOS and the slowdown dose exist but it's barely noticable.
Default install and I no longer enjoy tinkering with a device I rely on so heavily these days. I am sensitive to things slowing down and I tend to find combinations that cause them pretty often.