My understanding is that these old systems were slow because the window system was a compositing window system, and old hardware doesn’t keep up well with it. U

My understanding is that these old systems were slow because the window system was a compositing window system, and old hardware doesn’t keep up well with it. Using a compositing window manager has a lot of advantages, though, which is why most modern window managers work that way. Windows Vista also introduced a compositing window manager IIRC, and it was also horribly slow on old systems.