Why Is My Network Slow?
A slow network almost always comes down to one of a few causes: a duplex mismatch, Wi-Fi interference, congestion/saturation, a slow DNS lookup, or faulty cabling. The trick is measuring where the slowness actually is before changing anything.
The usual suspects
- Duplex mismatch — one side half-duplex, the other full. Causes collisions and terrible throughput. Check interface counters for late collisions.
- Wi-Fi interference / weak signal — overlapping channels, distance, walls. Wired always beats flaky Wi-Fi for diagnosis.
- Congestion — the link is simply full. One host doing a big upload can starve everyone.
- DNS latency — pages feel slow to start loading, but downloads are fast once begun. Classic DNS symptom.
- Bad cable / dirty fibre — errors force retransmissions. Check the interface error counters.
How to diagnose it
ping <gateway> -t # is latency high or jittery on the LAN? tracert <site> # where does latency jump? show interfaces # (switch) look for errors, collisions, drops nslookup <site> # is DNS resolution slow?
Compare wired vs Wi-Fi, and one device vs another. If only one device is slow, it is that device; if everyone is slow, it is the network or the internet link.
Reading interface errors
On a Cisco switch, show interfaces reveals the physical-layer truth: rising CRC errors point to cabling/interference; late collisions point to a duplex mismatch; output drops point to congestion. Fix the counter that is climbing.
Frequently asked questions
What causes a slow network?
Most commonly a duplex mismatch, Wi-Fi interference or weak signal, link congestion, slow DNS, or faulty cabling causing retransmissions. Measure where the slowness is before changing settings.
How do I test if my network is slow or my internet?
Ping your gateway and test a large file transfer on the LAN. If the LAN is fast but internet is slow, the issue is your WAN link or ISP, not your local network.
What is a duplex mismatch?
When one end of a link runs full-duplex and the other half-duplex. It causes collisions and severe slowdowns. Setting both ends to auto-negotiate, or both to the same fixed setting, fixes it.
Why do web pages start slowly but download fast?
That pattern points to DNS latency — the name lookup that must happen before the connection is slow, but the transfer itself is fine once it begins.
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