Short answer: first make sure the test is fair: a computer plugged into the router with a cable, nothing else downloading, and a couple of different test servers. If that wired result is close to your plan, the shortfall is your Wi-Fi. If the wired result is slow too, something in the chain between the provider and your computer can’t keep up, and on gigabit and multi-gig plans that’s usually a port, a cable or the modem. Find it link by link before you call your provider.

Step 1: run a fair test

A speed test measures the connection between your device and a test server, so a lot can skew it. Speedtest’s own guidance is that phones and computers have very different radios and some can’t measure your full speed, that nearby servers are usually faster and it’s worth trying several, that browsers can give different results on very fast connections, and that you should close other downloads and video calls before testing.

So for a fair baseline:

  1. Use a computer connected to the router by Ethernet, not a phone on Wi-Fi.
  2. Pause other traffic: downloads, backups, streaming, game updates.
  3. Test two or three servers, at a couple of different times of day.

Step 2: wired fine, Wi-Fi slow?

If the wired test is close to your plan, your internet connection is doing its job; the loss happens over Wi-Fi. Speedtest notes that the router itself may not support your plan’s full speed over Wi-Fi. Work through slow Wi-Fi for the fixes, and remember a device’s own Wi-Fi radio caps what it can reach; router speed ratings explained covers why.

Step 3: wired is slow too? Follow the chain

Wired speed is only as fast as the slowest link between the street and your computer. One 1 Gbps link anywhere in that chain caps a faster plan. Check each one:

The modem

Your modem has to be rated for your plan. Xfinity, for example, requires an owned modem to support your plan’s speed, and certification varies by provider: NETGEAR’s CAX30S modem-router is certified on Xfinity only up to 800 Mbps, but up to 1 Gbps on Spectrum and Cox. An older modem on a newer, faster plan is a common culprit; best cable modems lists current models by plan speed.

The router’s internet port

A router can’t pass more than its internet (WAN) port allows. gFiber recommends a router with a 1 Gbps internet port for its 1 Gig plan and a 10 Gbps internet port for its multi-gig plans. If your plan is faster than 1 Gig and the router’s internet port is 1 Gbps, that’s your ceiling. The best routers for multi-gig internet are sorted by plan tier.

Your computer’s Ethernet port

If your computer’s Ethernet port runs at 1 Gbps, the test can’t show more than about a gigabit however fast the plan is. On a multi-gig plan, check the port speed in the computer’s network settings and, if needed, add a faster adapter: see the best 2.5G and 10G Ethernet adapters.

The cables and anything in between

Multi-gig ports adjust their speed to the slowest end and can drop to 1 Gbps or even 100 Mbps if the cable, a port or the device at the other end can’t do more. A single old cable or a 1 Gbps switch in the path is enough. Existing Cat5e and Cat6 handle 2.5 and 5 Gbps, but 10 Gbps over Cat6 is only specified for 55 to 100 metres, so long runs matter at that speed. If you need a switch, the best multi-gig switches keep the faster speed; multi-gig Ethernet explained covers the standards.

Step 4: still slow? Restart, then call your provider

Restart the modem, then the router, which makes the modem set up a fresh connection with your provider. Speedtest also suggests checking that the router’s QoS (traffic shaping) features aren’t limiting the test. If a fair wired test is still well short of your plan with equipment that’s rated for it, it’s time to contact your provider, with your test results in hand.

If speed tests look fine but calls and games still stutter when someone else is busy, the problem is latency under load, not speed: see bufferbloat.