How come some twice exceptional children (i.e., intellectually gifted children who have some form of disability) have high iq, but low processing speed? Isn't processing speed a factor of intelligence? If not, why do iq tests have a time limit?
Being twice-exceptional (2e) myself I can tell you what I know. As another reply indicated the appearance of "slowness" is an effect of the observer's perception. This is true because of a fundamental difference with the mental processing techniques adopted by twice exceptional, gifted people and people described by the autism spectrum.
For most people mental processing inclinations involve a bottom-up orientation, which is like stacking Lego blocks to build up coherent thought. While 2e people, and their like, are inclined towards a top-down mental processing orientation.
Bottom-up processing is immediately productive because each block added builds the resulting product. Top-down processing is not particularly productive because it doesn't really make things--instead organizes things logically so results are arrived by removing irrelevant or senseless aspects of structural relationships (basically reclassifying info sets towards task relevancy, i.e., grouping and ordering things). (Catron & Wingenbach, 1986)
So basically 2e kids design systems when needing to produce results. Mainstream western culture places greater value on products (visual materiality) than processes (incremental materiality) in terms successful activity, particularly since we are addicted to immediate gratification. Therefore our value system allows the appearance of "doing nothing", thus organizing information for systemic comprehension is not valued do to its lack of visual materiality (you can not see it) which is attributed to being unproductive or slow.
Furthermore, to specifically answer your question: A high IQ does not specifically indicate lightning fast thinking abilities; instead it shows a person's aptitude. The tests original intention was not to sort people by levels of intelligence but instead identify those who are generally less informed than their age peers. The association is referred to as aptitude which can be described as their intellectual attitude or their inclinations towards being apt to learn (apt-attitude, aptitude). Speedy thinking is in this way an accidental symptom.
This is not an answer to your question but you probably want lookup Howard Gardner and his theory of Multiple Intelligence (MI).
The basic proposition here is that the IQ test (origins in the work by Alfred Binet) is not a wholistic measure of intelligence and that it measures the ability of one to think in a certain expected way. Gardner proposes that there are many ways to think (and thus how one learns) and he comes up with 7 distinctly different types of intelligence (his later work adds more).
I'm not familiar with your child subjects, but in comparison if you measure IQ of some Autistic children they may measure extremely high IQ, but may be perceived as slow or unresponsive to other tasks like social interaction (or low "EQ"). This does not mean that they are either intelligent or not intelligent, just that they think in a different way than the observer.
Speed (and age) is a factor of intelligence, but irrelevant if the underlying measure is fundamentally flawed. For example, you can be quick at maths or slow at maths, this will just tell you if you are intelligent at mathematical thinking. But it will not tell you is your are "intelligent" because you could be quick to learn music rhythms (thus musically intelligent) but this is not measured in IQ.