Only Getting 400 Mbps on Gigabit Internet? Causes and Fixes

What 400 Mbps Means on a Gigabit Plan

A gigabit plan is usually advertised at up to 1,000 Mbps, while an internet speed test measures the throughput available between one device and one test server at a specific time. Ethernet and network protocols create overhead, so a wired result below 1,000 Mbps can be normal. However, a stable result near 400 Mbps is low enough to justify troubleshooting, especially when the test is performed with a modern computer connected directly to the router.

First, confirm that the test reports Mbps, not MB/s. A download rate of 400 Mbps is approximately 50 MB/s. Also compare several test servers and run tests at different times. One result should not be treated as proof that the ISP is delivering only 400 Mbps.

Common Cause: Wi-Fi Capacity and Signal Conditions

Wi-Fi is often the main reason users see about 400 Mbps on gigabit broadband. The actual result depends on the Wi-Fi standard, channel width, signal strength, interference, distance, walls, client hardware, and the number of nearby networks. A device may show a high theoretical link rate while delivering much less usable throughput. Older Wi-Fi 5 equipment, crowded 5 GHz channels, and 2.4 GHz connections are particularly likely to limit performance.

How to check it

Run one test beside the router and another in the usual work area. Check the device's negotiated Wi-Fi link speed, switch to a clean 5 GHz or 6 GHz band when supported, and compare the result with a wired test. If Ethernet reaches much closer to the expected speed, the broadband service is probably working and the wireless network is the constraint.

Common Cause: Ethernet Link Negotiation

A wired connection can still be limited if the computer and router negotiate a lower link rate. Damaged cables, poorly terminated connectors, old switches, or a port that supports only 100 Mbps or 1 Gbps can affect throughput. A gigabit Ethernet link commonly delivers less than the line rate because of protocol overhead, but a negotiated rate of 100 Mbps or an unstable connection indicates a hardware or cabling issue.

How to check it

Inspect the operating system's Ethernet status and confirm a 1.0 Gbps link, then connect the computer directly to the router with a known-good Cat5e or better cable. Remove intermediate switches and powerline adapters during testing. Replace the cable if the link repeatedly drops, renegotiates, or reports 100 Mbps.

Common Cause: Router, Modem, or Gateway Limits

Some routers and modem-router gateways can advertise gigabit ports but struggle to route gigabit traffic when features such as deep packet inspection, parental controls, traffic analysis, or security scanning are enabled. Older hardware may also have a slower processor, limited NAT performance, or firmware problems. A router with only 100 Mbps LAN ports cannot deliver a gigabit connection to wired clients.

How to check it

Review the specifications for WAN and LAN throughput, update the firmware, and temporarily disable intensive security or traffic-management features for a controlled test. Test with one wired computer while other clients are idle. If the router has a built-in speed test, compare it with a client-based test; a large difference may identify the router or local network as the bottleneck.

Common Cause: Device Hardware or Software

The testing computer may be unable to process gigabit traffic efficiently. Older processors, limited memory, power-saving modes, outdated network drivers, antivirus inspection, browser extensions, and background downloads can all reduce the measured speed. Some laptops also use lower-performance wireless adapters or switch network modes when running on battery power.

How to check it

Test with a recent computer, close cloud-sync and streaming applications, restart the device, and update the network adapter driver and operating system. Compare a browser-based test with a reputable standalone test application when available. If one device reaches 900 Mbps while another remains near 400 Mbps, focus on the slower device rather than the ISP connection.

Common Cause: VPN, Proxy, and Security Routing

A VPN encrypts traffic and sends it through an additional server, which can reduce throughput because of encryption overhead, server distance, or VPN capacity. A proxy, secure DNS application, endpoint firewall, or managed work-device policy can produce a similar effect. The reduction may vary by protocol and destination, so a single test through the VPN can be misleading.

How to check it

Run a comparison with the VPN and proxy disconnected, provided this is permitted on the device and network. Keep security protection enabled unless a brief controlled test is safe and approved. If speed returns when the VPN is bypassed, try a nearby VPN location, a faster protocol, or a different provider rather than changing broadband equipment first.

Common Cause: Test Server, Time of Day, or Network Congestion

Speed tests measure a path to a selected server, not an abstract maximum for every website. A distant or busy test server, congested peering route, evening demand, or heavy traffic from another household device can reduce the result. Downloading from a single website can also be slower because that site's server or delivery network is busy.

How to check it

Use several nearby and geographically different test servers, repeat the tests during quiet and busy periods, and pause large downloads or streaming sessions. Run three tests and compare the median rather than relying on the highest or lowest value. If only one server is slow, the problem may be the test path rather than the access line.

Common Cause: ISP Provisioning or Access Network Issues

An incorrect service profile, optical signal problem, cable signal issue, damaged outside plant, or local ISP congestion can cap performance. A modem may also have outdated firmware or channel errors. These conditions are more likely when a direct wired test remains near 400 Mbps across multiple reliable servers and at different times.

How to check it

Record the test time, server, connection method, device, download result, upload result, and latency. Check the modem or optical terminal status for error messages or abnormal signal readings, but do not change provider settings without guidance. Contact the ISP and provide the documented wired results. Ask the provider to verify the service profile, equipment compatibility, line or optical levels, and local congestion.

A Practical Troubleshooting Sequence

1. Restart the modem, router, and test device, then wait for the connection to stabilize.
2. Connect one modern computer directly to the router with a known-good Ethernet cable.
3. Confirm a 1 Gbps Ethernet link and verify that the router's WAN and LAN ports support gigabit speeds.
4. Pause VPNs, proxies, cloud synchronization, downloads, and other high-bandwidth activity.
5. Run multiple tests against different servers at more than one time of day.
6. Repeat the test over Wi-Fi to separate wireless performance from broadband performance.
7. Contact the ISP if wired results remain consistently low after local equipment and test-server variables are excluded.

Ways to Improve Gigabit Download Performance

• Use a modern Wi-Fi 6 or Wi-Fi 6E router and a compatible client when wireless gigabit performance is important.
• Place the router in a central, open location and select an appropriate 5 GHz or 6 GHz channel.
• Use Cat5e or better Ethernet cables and avoid unnecessary powerline or low-speed network adapters.
• Update router firmware, modem firmware, operating systems, and network drivers.
• Disable or tune bandwidth-intensive router features if testing confirms that they reduce throughput.
• Use wired Ethernet for large downloads, gaming updates, and other activities that need consistent speed.
• Compare download, upload, and latency results because a speed increase with high latency may indicate congestion rather than better service quality.

For additional context, compare results with the guidance from your speed test provider and your ISP's equipment requirements. The goal is not to force every test to show exactly 1,000 Mbps, but to identify whether the missing throughput comes from Wi-Fi, local hardware, the test path, or the provider network.