I did this once, and it was a blast! Time yourself, and don't try to rush it. I think it would be better to give a compelling reading of a snippet than to rush through the whole thing. My story was short enough I didn't have to worry about time, and I'm not sure what I would have done had I had to cut it short. For that case, I think I would have read the last half, and maybe given a "our story so far" synopsis for the first 15 or so seconds of my time.
An aside, this reading was done as a conference call, and we could hear the other callers. One story was, shall we say, trying too hard, and when it was done all the other listeners were breathless. "Exquisite!" "Incredible!" "Beautiful and you are a beautiful person!" while I stiffled a "Lol, whut?" Then my story, which had death in the first sentence! Action! Relatable characters! A Vonnegutian/O Henrian gobsmacking ending! was met with "Huh." "Int. . . eresting." "Thanks." Oh well, you can't win them all.