KDE can be configured to run with pretty minimal overhead, but it takes quite a bit of knowledge about how the DE works. After disabling the fancy compositor effects though, I think I'm comfortable saying that it's more responsive than my i3wm setup on a 13-year-old x201. Not a knock against i3, as it's a much better tiling WM than KDE ever will be, but I really think the resource requirements aren't the far off from one another.