API Latency Debugging: Step-by-Step

Laggy endpoints hurt UX. Diagnose precisely.

1. Measure Baseline

Collect:

• P50 / P95 / P99 latency
• Error rate
• Request rate

2. Break Down Timing

Enable server timing or instrumentation:

DB=40ms | cache=5ms | render=60ms | network=20ms
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3. Check the Critical Path

Remove sequential awaits that could run in parallel (Promise.all).

4. Database Profiling

Log slow queries > 100ms. Add EXPLAIN for outliers.

5. Cache Strategy

• Layer 1: in-memory (hot keys)
• Layer 2: Redis for shared
• Invalidate on write or TTL-based

6. N+1 Detection

Look for many small similar queries. Batch or prefetch.

7. External Calls

Wrap in timeout + circuit breaker. Measure each provider’s latency separately.

8. Payload Size

Check response bytes. Remove unused JSON, compress selectively.

9. Re-Test

Compare new latency histograms. Document improvements.

10. Guardrails

Add SLO alert (e.g., P95 > 300ms for 5m) to catch regressions.

Consistent latency builds trust.