Minecraft wiki entry for Hopper, associated text: Hopper
A sequence of three or more hoppers, each pushing items into the next, is called a hopper pipe. Working horizontal hopper pipes simply push items into each other at the expected rate of 2.5 items per second, but vertical hopper pipes are more complicated, as the hoppers are trying both to pull and to push. When a vertical pipe pulls from a single container, it simply transfers items at 2.5 items per second because the transfer rate is limited by the first hopper pulling items from the container. If a stack of items is in a vertical pipe, the items can be transferred twice as fast, because the hopper with the item stack is pushing items down while the hopper below it is also pulling items down.