A 68-year-old woman with COPD (FEV₁ 35% predicted) undergoes elective hip arthroplasty under general anesthesia. Intraoperatively, she receives fentanyl 150 μg IV and vecuronium 8 mg for induction and paralysis. At the end of surgery, she is given neostigmine 5 mg IV with glycopyrrolate 1 mg IV for reversal. In the recovery room, 2 hours post-extubation, she develops progressive respiratory depression (RR 6/min, SpO₂ 85%), pinpoint pupils, and hypotension (BP 85/50). Naloxone 0.4 mg IV is given with initial improvement, but respiratory depression recurs 45 minutes later. Which pharmacokinetic property of fentanyl best explains this recurrence?