1) As soon as reporting requirements get serious, you build a data warehouse. Because odds are, the client will want to combine data from multiple systems in their reports anyway. If not today, then they will tomorrow.

2) such reports never need all the data, it's mostly about top N volume queries or month-over-month performance data. When a reporting application does query all the data, it's because it's building its own data warehouse so the query usually happens only once per day, at a specific time, which means the load is entirely predictable.