The difference being that in the one of those cases you still need to open a browser instance before you are able to browse the web.

Firefox is absolutely unusable on that PC. Falkon runs much better.

Also I don’t think KDE even uses more RAM than other DEs that are designed to be lightweight. Last time I compared, it used the same or less memory as LXDE.

Firefox without any website loaded uses more RAM than a full Plasma session.

XFCE is great for mid-range old devices, and LXQt is great for dogshit old devices.

What’s this device in your scale from old doghit to old mid-range?

Runs a full Plasma session just fine. The problem isn’t the desktop, it’s the web browsers, especially Firefox. Falkon runs OK.