A family member has dyspraxia (movement disorder) plus cerebral palsy. The brain damage from these disease/disorders can affect neurotransmitters from what we've been able to understand. The normal method of brain activity is either delayed, or needs to divert to healthy transmitters. Consequently, his conversation is deeply affected: Oftentimes understanding the discussion, but unable to answer quickly.
I probably have this in error, but it's easy for me to understand this way, and handle with patience.