I did long distance trips wih a friend and he always claimed his bike was faster than mine. I contended that the difference was that he weighed 15 lbs more than I did. We switched bikes on a long downhill section of road and he was faster on my bike as well.
I would be curious as to the gearing on both bikes. The old one may have better gears for your style of riding. In the old days when I was stronger and lighter I could drive a 60 tooth chainring but now I use a 52 tooth chainring on my road bikes as with a 60-tooth one my cadence would be greatly reduced as would my average speed.