I just found this: www.med.umich.edu/pmr/acal/speed.htm

They are doing a research project on thinking speed. It says "Thinking speed, or processing speed, is a term that describes how quickly an individual can mentally process information. Processing speed has been shown to be closely connected to aspects of memory and intelligence and is sensitive to many kinds of brain dysfunction as well as medication effects. Traditionally, processing speed is measured by timing how quickly a child can use their hands to do a paper and pencil task. This type of measurement is obviously not appropriate for children with significant motor impairments. In contrast, our processing speed studies examine the length of time a picture needs to be shown in order for a child to accurately perceive it..."

and "Just because you move slowly, doesn't mean that you think slowly!"

I am happy that someone is looking at this. My twice-exceptional son with motor dyspraxia and hypotonia is sometimes a little slower than other kids physically but I never thought it affected his actual thinking speed.