I use twm. It's pretty lightweight, it doesn't conflate "being on top of everything else" with "focus", and it's highly customizable. Oh, you can also handle ongoing-process-icons in a sensible manner that doesn't eat up screen space.

I use Awesome3 on an old (P3-850, 128mb of RAM) laptop and it screams like the wind. It's incredibly configurable and is great for getting out of the way. Also, once you try tiling, you won't want to go back.

I also use Xmonad now and again on another old box, but that mostly runs headless.