Event loop blocking misdiagnosed as downstream service latency

Latency spike appears to correlate with outbound request timing, leading to false diagnosis of downstream service issues. However, downstream service shows no latency increase (API service: 698 microseconds vs SSR perspective: 397 milliseconds). Root cause is event loop delay preventing response processing callback execution, not downstream performance.

When diagnosing latency issues, check event loop metrics BEFORE investigating downstream services. Compare server-side request timing with distributed trace timing from downstream perspective. If event loop delay is high but downstream service metrics are normal, focus on event loop optimization rather than downstream investigation. Use distributed tracing to identify where time is actually spent.