Slow internet: 12 common causes and how to fix them

By The SpeedFibre team, updated . 3 min read.

Slow internet: 12 common causes and how to fix them
In short Start with a test over a cable: if it is good, the problem is Wi-Fi or the device; if it is bad, it is the router, the line or the network. Restart the router, stop hidden usage and test at several times before calling your provider.
Contents
  1. Step 1: measure, then isolate Wi-Fi
  2. Causes inside the home
  3. Causes in the router and line
  4. Diagnosis table
  5. When to contact your provider

A slow connection can come from a dozen places. Rather than trying everything at random, follow this method: it isolates the cause in about fifteen minutes, with a speed test as evidence.

Step 1: measure, then isolate Wi-Fi

Run a test from the device with the problem, then a second one over an Ethernet cable connected to the router.

  • Cable good, Wi-Fi bad: the problem is in your home (Wi-Fi or device). See causes 1 to 6.
  • Cable bad too: the problem is upstream (router, line, network). See causes 7 to 12.

Causes inside the home

1. Wi-Fi distance and obstacles

Load-bearing walls, concrete floors, mirrors and aquariums absorb the signal. Each wall can halve the speed. Fix: move the router, use 5 GHz, or a Mesh system. See improving your Wi-Fi.

2. Crowded 2.4 GHz band

In apartment buildings, dozens of networks share the same channels. See 2.4 GHz or 5 GHz.

3. Hidden usage

Windows updates, consoles, photo backups, cameras, background downloads: one device can hog the connection and send ping soaring.

4. Limited device

An old phone, a computer with an old Wi-Fi card or a 100 Mbps Ethernet port: the device tops out before the connection does.

5. Software

VPNs, antivirus inspecting traffic, browser extensions, malware. Test without the VPN and in a private window.

6. Damaged Ethernet cable

A crushed or old-category cable can limit you to 100 Mbps or cause errors.

Causes in the router and line

7. Router overheating or needing a restart

Unplug it for 30 seconds. Keep it in the open, not in a closed cabinet.

8. Degraded fibre signal

Bent optical cord, dirty connector, blinking LOS light. See red LOS light and how fibre works.

9. Long or noisy ADSL line

Filter, extension, secondary socket: see slow ADSL.

10. Peak-hour congestion

If speed always drops in the evening, see why speed drops in the evening.

11. Data cap or throttling

On a 4G router or mobile plan, speed may be reduced after a certain volume.

12. Network incident

A local outage or roadworks. Your provider's social media and your neighbours are good indicators.

Measure your connection now. Start the test

Diagnosis table

Cable testWi-Fi testLikely diagnosis
GoodGoodOne-off problem or a specific website
GoodBadWi-Fi or device
BadBadRouter, line or provider network
Good by day, bad in the evening-Peak-hour congestion

When to contact your provider

After restarting the router and testing over a cable at three different times. Note the results, the router lights and the times. These details speed up handling by your provider's support. If only some services are slow, also check for throttling.

#WiFiDiagnosis#Ping#Troubleshooting

Sources and further reading

Frequently asked questions

Why is internet slow only on my phone?

Usually Wi-Fi (distance, 2.4 GHz band) or the phone itself. Compare with another device in the same spot.

Does restarting the router really help?

Yes. It clears memory, re-syncs the line and fixes a good share of temporary slowdowns.

How do I know if one device is using all the bandwidth?

The router interface often lists connected devices and their traffic. Otherwise, disconnect devices one by one and test again.

The SpeedFibre team Guides written and reviewed by a telecom engineer specialised in fibre optics (FTTH), with more than 8 years of hands-on network maintenance and measurement. Editorial policy.

Related guides