I've felt it on my Samsung Galaxy S4, and on my SO's Samsung Galaxy S3. Both of which are high-quality devices, not some random el cheapo smartphones. Over the course of two-three years they both went from smooth&crisp to annoyingly laggy to (in case of S3) pretty much unusable even for making phone calls.

Now in this story I'm just an alchemist with no experimental controls, but then again, the experience is not isolated. I'm not saying it's purposeful software slowdowns - maybe it's general bloat + flash memory wearing out? I don't know. But seeing as others report similar cases too, something is going on.