Why are "itch signals" transmitted so slowly from the skin to the human brain?
What causes the "itch" fibers to transmit signals to the brain so slowly? Is there thought to be some evolutionary benefit to such slow conduction, considering that the speed of most signals from skin to brain are transmitted so much faster?


The interview most likely refers to the findings of Schmelz et al., J. Neuroscience, 1997: Specific C-receptors for itch in human skin

In essence they found tested the sensitivity of unmyelinated nerve fibers (note: myelination also increases transmission speed) towards distinct stimuli. They found that some of those fibers would selectively respond to stimuli which would be perceived as itching (such as histamines). They find that:


  This slow conduction velocities attributable to small axon diameters ...


Note that since they don't need to, they also don't speculate on the physiological reason, why itching should be processed by slower fibers (and some potential explanations will be recognized quite well by readers of their article). 

Please also note that the original claim, from 1997, however has been softened with the years, and there appears no strict separation been stimuli like "hot", and itching, in terms of distinct fibers and transmission speeds; e.g.: Davidson et Giesler, Trends in Neuroscience, 2011: The multiple pathways for itch and their interaction with pain