Cart endpoint p99 regression

After a deploy at 10:00, GET /cart p99 regressed from 90ms to 700ms; p50 went from 40ms to 60ms (mild). Throughput and error rate are unchanged. Distributed traces for slow requests show the time is spent in many small, sequential database calls, a slow request makes ~150 round-trips to the DB, each ~3ms, while a fast request makes ~5. The deploy 'added per-line-item promotional pricing.' Carts vary in size; big carts are slow, small carts are fine. Triage and explain the regression precisely.