on my eee pc ro051bx (amd c-60 1.3mhz 1gb ram) unity 3d & unity 2d work the same speed.
unity 3d is somewone say just a compiz pluggin, and unity 2d is combination of unity 2d panel, unity 2d launcher and unity 2d places, so unity 2d run multiple processes and is compicated then unity 3d.
you can run every part of unity 2d independed foristence:
you can run in openbox/gnome (after install openbox you get openbox and openbox/gnome session in lightDM), unity 2d panel and cairo-dock if you did't like unity launcher, and run compiz in that.
also, you can run unity 2d launcher in gnome-shell if you want...
unity 3d and unity 2d is complitle diferent things...