If your /proc/cpuinfo says Intel(R) Atom(TM) CPU N270 @ 1.60GHz or something similarly weak, run xfce. While “xfce” may be a stupid and unpronounceable name, it is an excellent software suite. Do not run Unity, even with graphical effects disabled as completely as possible.
Actually, never run Unity under any circumstance. Or Windows. In fact, it might be best to throw away your computer and try something less exasperating, like having some children.