A true classic

I think it was on one of the Half-Life 2 developer commentary where they mention that the made the NPC move faster than your walking speed, but slower than your running speed, so that you are able to catch up with them if you stay behind to look at something. If they move at your running speed, you are kinda forced to follow them all the time, and any obstacle will separate you more and more from the NPC that you are supposed to escort.