I think that rule is probably alright for the average, non-exerciser, when they hike, but it sure is far off for well-conditioned persons. Part of your hiking equation depends upon the terrain too. If it is rocky and poor footing, then slowing down occurs a lot, especially for non-fit persons. I lived in Colorado and hike up Long's Peak a few times. Most people left at 4 AM to reach the top by noon and then returned by 8 pm. My ex-girlfriend and I would leave at 8 AM or later and be there by noon, stopping to eat and take pictures along the way of animals. We didn't hurry by our standards and were up there, rested for half an hour, looking at the scenery, and back by 4 pm. So, it seems to me that our pace was not slowed very much. I bet we slowed only 10-15% even though it was relatively steep in some sections and semi-steep in others. To a degree the severity of performance decline is due to general conditioning and to a degree specific conditioning. A lighter person climbs faster too, just like in cycling lighter people have advantages. I suspect that light, but strong runners slow far less than heavier runners that are of equal ability on the flats.