I was running some new tires in the mud and all the rubber hairs on the new tires seemed to be slowing me down (there were a ton of them - GK SS). The mud was one degree more sticky. Once I was out of the mud it was not noticeably different. So if I was doing a century with some mud, personally I'd ride or clip the hairs off those tires. Most tires have too few hairs to matter.