While some studies seem to indicate a slight improvement in surface turbulence due to the slime on a fish, the real purpose of the slime is clearly protection. This slime layer provides a layer of protection from infection and parasites for the host fish. This is why it is so important to use proper gear and handling during catch/release to protect this slime layer. Disturb it and you are not making the fish swim harder, you are potentially killing the fish.
http://www.aquaticcommunity.com/aquarium/slimecoat.php
http://www.arkansasstripers.com/fish-slime-coat.htm
http://www.petplace.com/fish/why-are...imy/page1.aspx