Desktop is taking a bit to render after a reboot (which I need to do very rarely, admittedly), apps definitely got slower to open, a small delay in actions here and there - it started feeling a bit sluggish compared to before and it felt like it's deteriorating over time - but slightly.

I think it's very likely due to OS updates, like other people are commenting on. It probably doesn't happen to everyone, and maybe doing a clean reinstall helps (I tried all the standard stuff - rebuild caches etc).