I agree with the previous post. It's all about temperature differentials and contact time. If you have ice-cold water moving through your chiller at the speed of sound, it won't have much time to transfer the heat from the wort.
An anecdote may help clarify. I bought MW's entry level 25' copper wort chiller two years ago. It's a good unit, but hard to get the hose clamp union where the vinyl hose meets the copper chiller to stop leaking. This summer, when I went AG and moved my operation outside, I decided to refit the chiller with compression / garden hose fittings. When I took the original clamps and hoses off, I noticed that there is a plastic tape inside the copper tubing of the chiller, evidently running the full length. I was quite puzzled by this, thinking that it could only slow the water flow down, and that was bound to be a bad thing. I e-mailed MW, and in the reply, they told me that the plastic tape is put into the tubing on purpose, and the purpose is to slow the flow down. The e-mail said that without the tape, it would actually take 15% longer to cool the wort. This ensures better transfer, not so much from slowing the flow, but in mixing the cold water as it flows through the tube, ensuring the maximum contact with the walls of the tubing. As everyone knows who's done chilling for any length of time, it proceeds more quickly the more you stir the wort. Think of the plastic tape as stirring the water inside the copper tubing, as you stir the wort outside. This maximizes the temperature differential on both sides as the liquids exchange heat through the copper.
{NB: I'm just SURE some engineer can explain this more concisely, in a few well-chosen equations that I would never understand. I've tried to give my own explanation, as best I understand the process.}