How to diagnose slow network performance

Question

QA Hub Editorial · Accepted Answer

Short answer

Diagnosing slow networks requires isolating whether the issue is bandwidth exhaustion, high latency, packet loss, or application-level inefficiency.

Measure baseline speed: Use tools like iperf3 or speedtest to measure actual throughput against expected capacity.
Check interface utilization: Review switch and router interfaces for high utilization or errors.
Test latency and loss: Run ping and traceroute to identify where delays or drops occur.
Analyze traffic patterns: Use NetFlow or packet captures to spot unusual bursts or misbehaving applications.
Verify duplex settings: Mismatched duplex between devices causes collisions and poor performance.
Review QoS policies: Ensure critical traffic is prioritized and not starved by bulk transfers.

Test at different times to distinguish peak-hour congestion from persistent issues.
Check physical layer first: bad cables cause intermittent slowdowns.
Monitor CPU and memory on routers during high load.