Earlier quoted context omitted.
> Bump it to 3-4 years then. Bloated manufacturer updates combine with bloat in most popular applications and the regular web bloat to make the phone unusably slow after few years. My S3 is 4 years old now, and it is works perfectly fine. When it dies, I will most likely adopt one of my Lumia devices as main one, or will buy a 2nd hand Android device, instead of giving money to support bad OEMs
My SO's S3 is of more-less the same age and it's so slow that it's barely usable now. Still can't track down why - she is not a power user, she wasn't installing apps beyond the few things I installed her and the OS updates. My old S4, currently used by my brother, suffered the same fate, being slow even after a factory reset. I wonder where this comes from?
However do be aware that your perception of what is slow changes as you use newer devices which perform better, making older ones feel slower than they did before - much like a shiny new work PC makes your home PC feel slow (or vice-versa) - so that may partly explain it.
Finally, I've just breathed new life into my ageing Nexus 7 (2013) tablet by installing the custom ROM "AICP" on it, which has made a huge difference to its performance & battery life. I recommend giving it a whirl on your SO's S3 - download from http://dwnld.aicp-rom.com/?device=i9300 (though you'll need to read up elsewhere about installing custom ROMs if you've never done it before)