Went on an 18 mile ride yesterday.
The Continental Gatorskin Hardhsells are a lot smoother riding than my Marathon Plus tires. And despite missing a turn, by 100 yards, resulting in a turnaround, I shaved 10 minutes off the course. I rode the same route a week ago. So 10 minutes over 18 miles...I would consider that a big improvement and I am guessing it is all done to the tires. I think it took me from a ~10.8 MPH to a ~11.4 MPH average speed.
Will be doing a long ride today with some gravel thrown in and will see how that compares too.