Now that's a loaded question.
It's not an easy question to answer but I'm gonna go with ducks in a strong wind, so strong that it looks like they're hardly moving or when flying and twisting whistling wing fast dropping out of the stratosphere into your deeks. .
My shot averages are far better with dove than they are with waterfowl. For some reason ducks for me are more harder to hit. I have too many scratching my head moments thinking how did i just miss 3 of the easiest shot in the world. The only thing i can think of is its due to so many variables while in the duck blind. Many different species of duck that all fly at different speeds(effecting leads) and behave differently. Other thing is I've been shooting at upland game far longer than waterfowl.
I have always wondered if the speed of waterfowl loads throw me off as well. When shooting clays or doves im normally using loads that are the same FPS but waterfowl loads are so much faster. Maybe one day ill shoot a round of trap with my waterfowl loads to see if there's a difference.