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Why did my internet slow down? Diagnosis in 7 steps

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Everything was fine yesterday; today pages crawl and videos freeze mid-stream. There is no single answer to "why is my internet slow?" — but checked in the right order, you can narrow down the culprit in about ten minutes. The steps are deliberately ordered from easiest to most involved.

1. Measure first: how slow is "slow"?

Feelings mislead; numbers don't. Run the speed test and write the result down. Two things to keep in mind:

  • Your plan is sold in megabits (Mbps); download managers show megabytes (MB/s) — an 8× difference. Downloading at 12 MB/s on a 100 Mbps line is normal.
  • One test decides nothing; measure a few times during the day. Tests on this site are saved to your history — that is where the trend shows.

2. Take Wi-Fi out of the equation

The biggest source of slowness complaints is not the line but the wireless network. If you can, connect a computer to the modem with an Ethernet cable and repeat the test:

  • Fine over cable but slow on Wi-Fi → the problem is wireless: distance, walls, interference from neighbouring networks, modem placement.
  • Slow on both → continue to the next steps.

3. Check the other devices on your network

A TV streaming 4K, a phone uploading its cloud backup or a console pulling an update can swallow most of your bandwidth. Temporarily switch the other devices off (or drop them from Wi-Fi) and re-test.

4. Restart the modem — properly

Pull the plug, wait 30 seconds, plug it back. Modems left on for weeks slow down due to memory leaks and heat; a simple restart fixes things surprisingly often. If your modem is older than ~5 years and freezes regularly, consider requesting a replacement from your provider.

5. Watch the clock: congestion

If speed drops every evening between 20:00 and 23:00 and recovers after midnight, you are looking at regional congestion. The way to prove it is measuring at different hours. A consistent, documented evening drop is a valid basis for a complaint — the step-by-step process is in not getting your advertised speed.

6. VPNs, DNS and "security" software

  • If a VPN is on, turn it off and re-test: routing traffic through a distant country can halve your speed.
  • Browser extensions and some antivirus suites proxy your traffic — if in doubt, try a different browser or a private window.
  • Changing DNS does not "speed up the internet", but a broken DNS slows page openings; if pages start slowly while download speed is normal, look at DNS.

7. Still slow? Modem or line

If you got this far, the problem is either the modem or the line. Telling them apart at home deserves its own article: Is it the modem or the line? Continue there with the symptom → culprit table; and if the line turns out to be guilty, the measurements you collected in step 1 are already your evidence file.

The checklist

  1. Measure and know the number (Mb ≠ MB)
  2. Repeat over a cable
  3. Silence the other devices
  4. Unplug the modem for 30 s
  5. Measure at different hours
  6. Toggle VPN/extensions/antivirus
  7. Still bad → modem-vs-line isolation