I think the first thing to recognize is that there is absolutely no single measure of intelligence. IQ scores are seriously flawed and discredited. Human intellectual powers have many dimensions. Someone may be weak in speech and articulation but on the other hand be an absolute genius mechanically, artistically, or in some other aspect of human intellectual pursuit.
Discredited may be an overstatement on my part, but it is a flawed measure of someone’s complete abilities and strengths.
Reference: https://en.wikipedia.org/wiki/Intelligence_quotient#Validity_as_a_measure_of_intelligence
There are other criticisms in that section of the Wikipedia article too.