Why does Wi-Fi slow down?
Most of the moments we call “the internet is slow” are not about the internet at all but about the wireless network inside the house. Understanding why Wi-Fi slows down is half the solution; most cases are put right with a few simple moves.
Short answer
Wi-Fi slowness has four main causes: the distance and obstacles between you and the modem, channel congestion with neighbouring networks, the number of devices using the network at once, and the already limited speed of your subscription. A wired speed test is the quickest way to tell whether the problem lies in the network or in the service.
Diagnosis first: where is the problem?
The first step is identifying the culprit correctly. If you can, connect a computer to the modem by cable and run a speed test; if the wired speed is close to what your subscription promises, your internet connection is healthy and the problem is on the wireless side. If the wired speed is low too, the matter concerns the service provider.
The second clue is location: if the slowness occurs only in certain rooms, distance and obstacles are the likely cause; if it happens everywhere and at particular times, congestion or a service problem is more probable. If it happens on one device only, the fault may lie with that device rather than the network.
Distance and obstacles
A Wi-Fi signal is a radio wave, and every wall and every floor takes a piece of it; concrete, metal and water (an aquarium, a boiler) are the greatest enemies. A modem sitting inside a cupboard in the corner of the house is the most common and most easily corrected problem.
Move the modem as close to the centre of the home as you can, high off the floor and with space around it. In large or multi-storey homes a single modem may not reach everywhere; signal repeaters or mesh systems placed at several points come into play here.
Channel congestion and frequency bands
In blocks of flats, dozens of networks share the same air, and networks using the same channel slow one another down; it is like everyone speaking at once in a crowded room. Most modems choose the channel automatically, but sometimes moving manually to a freer channel makes a difference.
The bigger difference lies in the frequency bands: the 2.4 GHz band reaches further but is slower and more crowded, while the 5 GHz band is much faster with a shorter range. If your modem is dual-band, connecting devices close to it to the 5 GHz network produces a noticeable improvement in most homes. For the parts the modem and the router play in this, see the modem and router guide.
The age of your devices is part of this equation too: wireless standards develop over the years, and newer generations (seen on boxes under names such as Wi-Fi 5 and Wi-Fi 6) offer higher speeds and better handling of congestion in the same environment. A very old laptop or phone stays limited to the speed of its own older standard even on the best modem.
Device numbers and sharing the bandwidth
Your connection's speed is shared among everyone using the network at that moment. With 4K video on one device, a large game update on another and a video call on a third, slowness on the fourth is only natural. Smart televisions and cloud backups can quietly pull large amounts of data in the background.
Every device that stays connected without being used also takes a share of the modem's airtime. Keeping your network password strong prevents uninvited neighbourly guests from using your bandwidth as well.
Know too that the slowest link on the network can affect everyone: while a very old device is connected, the modem has to give part of its time to it at a low speed. Taking unused old devices off the network, and gathering low-priority devices such as smart home appliances onto the 2.4 GHz band where possible, lets your main devices breathe.
Service speed and the age of the modem
Sometimes everything is correct but the expectation is wrong: if your subscription is already limited to something like 16–25 Mbps, no modem setting will exceed it. Find out the speed your subscription commits to and compare it with the wired test result; if there is a clear gap, speak to your provider.
The modem itself ages as well: devices over five years old may not support current Wi-Fi standards. Restarting the modem regularly, installing firmware updates and, if necessary, asking the provider for a current model resolves most chronic slowness.
Checklist
- Run a speed test over a cable and compare it with your subscription
- Move the modem to a central, high and open spot in the home
- Connect nearby devices to the 5 GHz band
- Review the devices using the network and any background downloads
- Update the modem firmware, and ask for a replacement if it is old
- Make sure your Wi-Fi password is strong
Frequently asked questions
Why does restarting the modem help?
Restarting clears the connection records accumulated in the modem's memory and refreshes the channel selection. It is not a definitive fix, but it is a simple first step that clears most temporary blockages.
Why is my speed test result lower than my subscription?
On a wireless connection, distance, obstacles and current congestion naturally reduce the speed, and the location of the test server has an effect too. For a realistic comparison, run the test over a cable and several times at different hours.
Can my neighbour's network slow my internet down?
It cannot take your speed directly, but networks broadcasting on the same channel lower one another's signal quality. Changing channel or moving to the less crowded 5 GHz band reduces the effect.
Should I buy a repeater or a mesh system?
For weakness in a single room a repeater is an economical solution, though it can reduce the speed somewhat. In large or multi-storey homes, mesh systems installed at several points give a more balanced result. The need varies with the structure of the home.
This guide draws on the official support documentation of network hardware manufacturers and on the publicly available technical guidance of internet service providers.