I actually tend to describe processing speed in terms of the conditions that will generate the highest quality response, instead of attempting to develop analogies for the underlying learning profile:
If you give this child x % additional time (standard accommodations are +50, +100, or unlimited), the quality and depth of his/her response will skyrocket, and you will get to see the child who scores in the top 1 out of 100 (or whatever their level is) in problem-solving ability. If you restrict them to normal time limits, you will miss out on that child, and will instead see only their much lower (bottom 1 out of 100 or whatever it actually is) speed. Most teachers are motivated by seeing children perform well and demonstrate skills and knowledge. While extra time benefits most students, the magnitude of the benefit is not remotely comparable for a child with slow processing speed. Rarely do I have a teacher who does not get on board once this is explained to them.
This explanation also has the advantage of applying to all underlying causes of slow processing speed on a pragmatic level, without having to get into complicated explanations of the exact neuropsychological or motor processes involved. Which, for some teachers, is a bit eye-glazing.
