CPU spikes on report serialization

A Python (FastAPI/uvicorn) reporting endpoint's CPU climbs from 30% to 95% and p99 goes from 200ms to 9s starting at 10:05, while request rate is flat and DB query time is unchanged. A py-spy flame graph shows ~85% of on-CPU time in JSON serialization and a nested list-membership check (if x in big_list) inside a per-row loop. The only change this morning was a 'cosmetic' update that made each report include the full list of all related entities per row instead of just IDs, average rows-per-report didn't change, but the per-row payload and the related-entity list got much bigger. Triage and mitigate.