What do we know about schizophrenia and mathematics?
I spent times of the order of five to eight months in hospitals in New
  Jersey, always on an involuntary basis and always attempting a legal
  argument for release. And it did happen that when I had been long
  enough hospitalized that I would finally renounce my delusional
  hypotheses and revert to thinking of myself as a human of more
  conventional circumstances and return to mathematical research. In
  these interludes of, as it were, enforced rationality, I did succeed
  in doing some respectable mathematical research.


Seems like he himself is saying his research suffered while unwell and not staying in institutions. 

However, the standard deficit in IQ, of it seems around 10 points, may be inconsequential in someone like Nash. Even if the executive function and memory impairments are more significant.