Assuming turbulent flow on the cooling liquid, the big parameter is the surface area. The next is the delta T between the cooling liquid and the wort. If you flow slow, the cooling liquid will gain temp, and towards the end of the chiller will be close to the wort temp, so not much heat will be carried away on that section. If you flow fast you maximize the delta T along the whole length, and will chill faster, even though the water coming out seems cool. Keeping the wort moving by stirring or with a pump will also maximize the delta T near the chiller, as a layer of cool wort will develope near the chiller, and decrease the delta T.
So, get the most area, flow fast, stir or pump the wort to keep it moving. Counter flow chillers maximize the delta T. Plate chillers have very large area, and the liquids counter flow maximizing the delta T.