In my opinion it is reasonable advice to replace the SD Card with an A1 class or upgrade it to an USB solid-state drive (especially if swap is being used).
About the only thing that can wear out and slow down with age on a Pi is the SD Card. As the card gets older the wear-leveling logic makes write operations more costly. Note that performing a filesystem trim (if supported) may partially restore write speeds.
Another thing which could slow the Pi down over time is software bloat, especially in the web browser code base.