1) Remember it is total weight that matters (rider+bike+crap). So, depending on how much you weigh, the effect may not be huge. 12 lbs may only be a 5% effect and that 5% is only for very steep hills, not flats and it's a smaller effect on rolling hills.

2) Wheel weight makes almost no difference for the wheels commonly used.

3) What does make the biggest difference at lower speeds is better rolling tires (this is what many people think lighter wheels buys them is really this).

4) At higher speeds, aerodynamics makes the biggest difference (a good rider in an 70 lbs velomobile will be significantly faster than on an 18lbs bike on rolling hills). Likewise, having a drop bar bike is likely to have an advantage over a flat bar bike.