When I was touring on a loaded down mtb (60lbs total, bike and gear) in the rolling farmland of southern PA, I was flying at my road bike speed, going up and down hills around 25 mph. Although it took me a lot longer to get up to speed, I could maintain it well both up and down the hills. I think the heavier weight punched through the air better and maintained my momentum better. I think the added width of the panniers was not really a handicap because of the extra weight, FWIW.