This is just my opinion, not based upon any science, but...

I have two short, but steep, hills to and from work. When I am on my commuter bike with my 30 lbs of junk on the rear rack, the bike climbs like a dog. Whn I have all of my junk in a backpack/shoulder bag, it is much easier.

I do not know why this is, however, 30 lbs of crap in a should back often time sucks more that a sluggish ride of a hill.