While i don't always walk at a particular pace, stairs seems to make me go faster. Whether it's up or down, i almost always speed up. I was wondering about this, and came up with a couple reasons.
I know a kid that constantly shakes his head as he reads. I asked his father why. He has a problem with his eyes, that they cannot stay focused without moving. To compensate, he shake his head in the same pattern. Very interesting. His view changed, so he changed to make the view consistent.
When i am walking up the stairs, i am moving forward slowly. My normal perspective of the world passing by is no longer the same. Perhaps i go faster to make it feel the same.
Another thing is the effort involved. It takes more effort to move up. Speeding up makes it go away faster.
Also, moving up, my steps are usually more narrow than on flat ground. This gives me a desire to take two steps at a time. Moving two steps at a time, however, can be uncomfortable at a slower pace do to the stretch. This made me do it more quickly when i was younger. I may just be accustomed to taking them faster, even now that i generally take one step at a time.
Moving down the stairs has another reason. It is easier. Wheras going up is fighitng gravity, going down is working with it.
I also noticed that when i went down steps i took two steps at a time, in rhythm. This has slowed me down as there is a small wait time between each rhythm. This is noticeable when taking stairs quickly. By being concious of this, i eliminated that problem, and all the steps are in one continous stream. This itself lends to taking them faster, as that was the point in the first place.