I haven't noticed this at all. My phone is 4 years old and feels just as fast as always. My previous one I remember thinking how snappy it was after a factory reset when I sold it on. The one before that died. The one before that kept its speed but the battery turned to shit and I couldn't be bothered trying to get it changed. I'm just about to replace this one but it's still in great working order so one of my kids wants it as their first phone.

There was one Android device my wife had that got slower and slower and it was a known issue with the disk.