Use Linux/KDE. None of the gains from the switch to SSDs have been lost. Everything is instant even on an n100. You only need something more powerful for compilation, gaming, or heavy multimedia (like the 200 Mbps 4k60 video my camera produces, which isn't accelerated by most processors because it's using 4:2:2 chroma subsampling).
> Use Linux/KDE
I do. It's not without its problems but currently it's the least bad solution.
Xfce or LXQt are also great alternatives, blazing fast even on 15 year-old hardware. Old hardware can be slow for basic web browsing and multimedia (e.g. watching videos with modern codecs) but other low-level uses are absolutely fine.