A possible explanation for the correlation with reaction time is brain damage. If you are exposed to more lead as a child, that will damage both your motor cortex and higher cognitive functions, even if those aren't otherwise connected. Same mutational load. If you got a random mutation that slightly affects your neurons slightly less efficient (everyone carries such mutations, but our genetics is redundant enough to handle it.)

But I wouldn't expect a perfectly "normal", undamaged human to be a genius, but they would have superior reflexes. And there are tons of animals that are dumb, but have amazing reflexes.