If power was identical, what are you attributing the loss of speed to? Weight alone would only account for a small amount. In another thread, Merlin showed a .1 - .2 mph difference from an 8 lb difference or .4 mph on a 6% grade. Biggest difference would likely be position. Get your CX setup the same as your road bike and you would be much closer. Also, 1.5 mph difference isn't "super slow". Cruiser bikes are super slow.