From a sample size of one (me). Two rides on consecutive days on the same route of pavement with a few short gravel sections. Temperature and wind similar. Bike geometry also very similar.

Day 1--18.5 lb road bike with slick 25 mm tires; 60 km total distance. Average speed 27.7 km/h.

Day 2--24.5 lb CX bike with 35 mm city/touring tires, 60 km total distance. Average speed 27.7 km/h.

There you go. Proof positive that weight has absolutely no bearing on speed. Finally, we can put the issue to bed!