How to Test Speed at the Router vs. Modem

Why Speed Can Differ at the Modem and Router

A speed test at the modem measures the broadband connection before most home networking functions are applied. A test at the router measures the same ISP connection after traffic passes through the router, Ethernet cable, security features, and possibly Wi-Fi. The difference may be small on a modern wired network, but it can become significant when the router is overloaded, the cable is limited, or the test device uses a weak wireless connection.

For a meaningful comparison, use the same computer, the same speed test server, and a wired Ethernet connection whenever possible. Pause large downloads, cloud backups, video calls, VPNs, and other traffic before testing.

Common Cause: Wi-Fi Signal and Interference

Wi-Fi is the most common reason for a slower router-side result. Distance, walls, neighboring networks, Bluetooth devices, and crowded 2.4 GHz channels can reduce throughput or increase latency. A modem-side test made over Ethernet may therefore look normal while a phone or laptop connected by Wi-Fi reports much lower download and upload speeds.

Common Cause: Router Hardware or Configuration

The router may not have enough processing capacity for the connection. Older hardware can struggle with high-speed fiber or cable broadband, especially when traffic monitoring, parental controls, antivirus inspection, VPN services, or advanced firewall features are enabled. Incorrect QoS settings, outdated firmware, or a poor WAN configuration can also reduce performance.

Common Cause: Ethernet Cable or Port Limits

A cable or port can create a bottleneck between the modem and router. A damaged cable may cause retransmissions, while older Fast Ethernet ports may limit speeds to about 100 Mbps. Check the negotiated link speed in the router or computer interface. For higher-speed broadband, use a reliable Cat5e or better cable and Gigabit Ethernet or faster ports.

Common Cause: Device and Browser Limitations

The test device may be unable to measure the available connection. Older Wi-Fi adapters, low-power processors, background applications, browser extensions, and a busy operating system can produce inconsistent results. A single phone or laptop should not be treated as proof that the ISP connection is slow. Repeat the test with a capable computer connected directly by Ethernet.

Common Cause: Modem, ONT, or ISP Problems

A slow wired result at the modem or fiber ONT points closer to the access connection. Possible causes include signal issues on cable broadband, optical problems on fiber, congestion in the ISP network, service maintenance, incorrect provisioning, or a failing modem. If several wired devices show similar low speeds at different times, the router is less likely to be the primary cause.

How to Test Speed at the Modem

1. Ask other users to stop heavy internet activity, including streaming, uploads, downloads, and cloud synchronization.
2. Connect a computer directly to the modem or fiber ONT with Ethernet. Some services require a PPPoE login or a specific VLAN configuration, so follow the ISP setup instructions.
3. Restart the modem or ONT only if needed, then wait until the connection is fully online.
4. Run at least three tests using the same reputable speed test service and record download, upload, latency, and test time.
5. Disconnect the computer and reconnect the router before testing the router-side connection.

Some ISP installations do not allow a direct modem connection because the router handles authentication, public addressing, or managed service features. In that situation, use the router's built-in diagnostic tool or test a computer through a wired LAN port with Wi-Fi disabled.

How to Test Speed at the Router

1. Connect the test computer to a Gigabit or faster LAN port using Ethernet.
2. Confirm that the computer reports a link speed appropriate for the broadband plan.
3. Temporarily pause VPNs, security scans, downloads, and other applications that use bandwidth.
4. Run the same number of tests against the same test location used for the modem comparison.
5. Repeat the test over Wi-Fi from the same room where slow performance is reported, but keep the wired and wireless results separate.

How to Interpret the Difference

• Modem and router wired results are both slow: investigate the modem, ONT, line quality, service configuration, or ISP network.
• Modem wired speed is good but router wired speed is slow: check the WAN cable, router ports, firmware, router load, and security features.
• Router wired speed is good but Wi-Fi is slow: focus on signal strength, channel congestion, band selection, device capability, and access point placement.
• Download is normal but upload or latency is poor: look for upstream congestion, background uploads, bufferbloat, line noise, or ISP-side faults.

Optimization Steps Before Contacting the ISP

Update the modem and router firmware, replace questionable Ethernet cables, and confirm that the WAN and LAN ports negotiate at the expected speed. Move the router to a central, elevated location away from metal objects and interference. Prefer the 5 GHz or 6 GHz band when the device is nearby, and use 2.4 GHz only when range is more important than throughput.

Reduce unnecessary router features during testing, then re-enable them one at a time to identify a performance impact. If the router supports SQM or a similar traffic-management feature, configure it carefully to reduce bufferbloat without setting an unnecessarily low bandwidth limit. For large homes, use wired access points or a properly configured mesh system rather than relying on a distant router signal.

When to Contact the ISP

Contact the ISP when direct wired tests remain consistently below the expected service level, the modem reports signal or optical errors, the connection drops repeatedly, or several devices show the same problem. Provide test times, wired results, latency measurements, modem or ONT status information, and the router comparison. This evidence helps the ISP distinguish an access-line issue from a home Wi-Fi problem without assuming that every slow test has the same cause.