A 48-year-old woman on ICU is receiving a propofol infusion for sedation and a remifentanil infusion for analgesia. She is being prepared for a spontaneous breathing trial. The remifentanil infusion is stopped. Fifteen minutes later, the nurse reports the patient is writhing in pain (Critical Care Pain Observation Tool score 6/8). What pharmacological characteristic of remifentanil explains this rapid onset of pain?