I agree with the above post that processing speed is not a direct indiactor of ones overall intelligence. For example, multiple sclerosis patients often have significant impairments in processing speed which have little effect on their overall intellectual ability.
Also, all forms of processing speed are not equal in terms of where the processing occurs within the brain. One could be very fast on tasks which require subcortical to cerebellar connections, but may not be as quick on tasks which add a prefrontal component (i.e. the critical thinking part of the test). In the prefrontal cortex, being slow can mean that you have to use more of it because the information is not well encoded (dorsal), but it can also mean that the PFC (medial) is inhibiting your desire to finish the test quickly just to get wasted after your exam. A long way of saying that test taking speed does not give you a clear picture of what is happening, anatomically speaking.
I think there is something to be said about the efficiency of processing, which may be reflected by white matter tract density, but its very hard to assess this in a psychometric way, or on any given test.
Its important to consider that even our best measures of overall intellectual ability are not predictive of many important functional abilities in life. Theory of mind is one ability which has huge functional importance, but is not highly related to IQ. Think of kids with aspergers who can be extremely bright, have very good use of language in a technical way, but be impaired in the social and pragmatic sense of language.
