I have roughly a 2 mph long-term average speed difference on the road between my road bike and my cyclocross/gravel bike. But, there are a number of factors that make the difference.
Road bike is carbon-framed and weighs in at 15.1 pounds, all in. Gravel bike is aluminum-framed and weighs in at 21.5.
I also carry more stuff on my gravel bike -- a few more tools, twice as many tubes, more CO2 -- and a frame pack rather than a saddle bag, so that adds a couple pounds more.
Road bike rides on light wheels and 23c tires. Gravel bike rides on heavy-duty "light" wheels and 28c to 38c tires.
On the same roads, on easy solo zone 2 HR rides, I average a little over 19 mph on the road bike -- a little over 17 on the gravel bike.