I don't usually do the same routes with gravel and road tires so it's difficult to have hard data to back my claim. The only one I have is 1h 59m vs 1h 56m on a flat fully paved route. So not a big improvement, 2.5%?

However the bike feels lighter with road tires. At low speed I accelerate faster and I can keep a higher speed for a longer time. The 42 mm tires have a shallow sawtooth tread so they can't be as fast as the slick 28 mm ones.

And the bike is really lighter with road tires. One 42 mm tire is 600 grams, plus a larger tube. I don't remember the weight of the 28 mm tire, but it's less and the tube is smaller.