joe213 First off, XFCE isn’t any lighter than plasma, despite its advertising. If XFCE is lightweight enough for you, anything not running Gnome3, cinnamon, deepinDE, or budgie will do. If you want something more lightweight, you should do LXDE/LXQt or MATE, or a window manager like openbox, xfwm standalone, i3, dwm, etc. et. al.
You could try any of the lightweight Manjiro distributions (not strictly arch, but mostly the same in practice). If you’re an arch purist then I can recommend ArchLabs.