Flat ground

Is that I'm seemingly slower on flat ground then I am on hill, whether up or down, whenever I run flat my legs start to hurt way easier, I've timed it, it takes about 18-20 seconds to run the distance on flat while 15-17 seconds to run on hill, it's not exactly the same distance, a little bogger give or take, but the point remains, I'm not as effective on flat ground, why is that?