there's a ton of factors that once added up can be 2-5mph avg difference
all i know is that the speeds i can sustain on my dads new madone/ultegra are about 3mph faster on a 20 mi ride than my 1993cdale/ultegra. the biggest difference is felt in acceleration. on flats, its not as noticeable
so yes you'll be quicker on a road bike if its full carbon vs (im assuming) a steel cx bike w/ non road tires.