Not less smart, just less fast

The brain is organized in a fashion where basic motor and sensory functions are well represented, allowing for a significant degree of redundancy. Therefore, if a problem occurs in one of these regions, the brain can adjust for it.

This is less true for cognitive functions that are considered "higher" functions, such as executive function, attention, working memory and information processing. Symptoms in CFS such as brain fog and memory impairments fall under these higher levels of brain function. Although many patients find ways to compensate and may not notice their full impairments all at once, it gets more exhaustive for the patient as they have to exert increased amounts of energy to do the same mental tasks.