This was and still is one of the most disturbing symptoms of the withdrawl for me right along with the fuzzy eyes.
Cognitive Fog takes a very long time to fade away and it is a result of the receptors trying to stabilize in the brain. I haven't read of anything in particular that will aid or speed up the recovery from this very troublesome condition. I will end on a positive note though - time seems to provide more and more relief and a reduction in severity. Mine has slowly faded to a level that allows me to read, watch TV and type on the computer.
Good luck to all,