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.
The Fairphone part of that review is great but it also includes a lot of Greenwashing from short term garbage producers like Apple and Samsung in a very uncritical way, I can recommend it as a Fairphone review but be aware that the rest isn’t great!