I don't think this is categorically true; try applying the Yerkes–Dodson law -- if your level of attention/arousal is too high, you may be performing sub-optima

I don't think this is categorically true; try applying the Yerkes–Dodson law -- if your level of attention/arousal is too high, you may be performing sub-optimally, and a small amount of impaired executive function may actually improve performance for well-rehearsed tasks. Counter-intuitive, but true. See also: Ballmer peak.