A starting point is the full text chapter "The Electrodermal Response", describes electrodermal response as being based on:
Interest in the conductance between skin electrodes, usually placed at the palmar surface, arose because of the involvement of the sweat glands in this measurement. Since sweat gland activity, in turn, is controlled by sympathetic nerve activity, this measurement has been considered as an ideal way to monitor the autonomic nervous system
Given that it takes time for processes in the human body to occur, the time delay or latency for the electrodermal response is due to, (from the article):
If one assumes initial resting conditions, then
a sweat response consists of sweat rising in the ducts
, and correspondingly R2 slowly diminishes. The response latency is associated with the time required for this to take place.
Further information that may be of use:
"Electrodermal response propagation time as a potential psychophysiological marker." (Silva et al. 2012). (Unfortunately the article is behind a pay-wall)
An important point from their abstract:
So far, there is still no clear consensus regarding the relation between the specific responses of the autonomic nervous system activity, and the features typically extracted from the electrodermal activity signals.
In this paper we present an experimental setup and corresponding data analysis for electrodermal response propagation time measurement.