I disagree. With shallow grades, the speeds will be higher and more of your watts will go to fighting air resistance rather than increasing your elevation. Take the example of a 1% grade vs a 10% grade. To climb 1000m with a 1% grade, you have to ride 100km. For a 10% grade, you have to ride only 10km. You can't ride 10 times faster up a 1% grade than up a 10% grade so your climb rate will be lower on the 1% grade.