Before changing anything: measure the ceiling
The most expensive mistake is spending money on a new router when your line is slow. The first check isn’t about the WiFi, it’s about the connection:
- Connect the computer to the router over Ethernet.
- Run a speed test.
- Write down the result. That’s the real maximum your line can deliver.
If you don’t reach the contracted speed over Ethernet either, it isn’t the wireless network: call your ISP. Everything below assumes you get good speeds over Ethernet.
If you get 800 Mbps over Ethernet and 60 Mbps over WiFi, then we have a wireless problem. And then the order of causes is as follows.
Cause 1: congested 2.4 GHz channel
This is the most frequent cause, and the easiest to check. In 2.4 GHz there are only three channels that don’t overlap: 1, 6 and 11. If your router and your neighbours’ are all on channel 3, they get in each other’s way even when the signal is perfect.
Check: find out which channel your network is on with an analyser app and count how many networks are on that same channel.
Fix: set your 2.4 GHz to 1, 6 or 11, whichever is freest in your area, and measure again. The change is immediate and costs nothing.
Cause 2: devices on the slow band
If all your devices connect to 2.4 GHz because the router only broadcasts one name, you have the problem solved before you start: separate the bands and move whatever you can to 5 GHz.
Check: look at the network name each device is connected to. If they all end up on the same network that doesn’t distinguish bands, that’s the reason.
Fix: enable both networks with different names, as explained in 2.4 GHz or 5 GHz.
Cause 3: too many devices
A home router has a real limit on simultaneous clients, and once you exceed it it starts dropping connections. The typical symptom isn’t constant slowness but intermittency: fast, stop, fast.
Check: see how many devices appear as connected to the router and compare that with the model’s specifications.
Fix: make sure there are no ghost devices holding DHCP leases, or move to a device with more resources. The detail is in how many devices a router can handle.
Cause 4: insufficient coverage
A strong signal next to the router and a weak one in the next room is a classic problem. At that point the speed drops because the modulation is reduced.
Check: measure dBm in the affected room. Below -67 dBm, the speed suffers even though the connection doesn’t drop.
Fix: changing channel can help, but what actually solves it is extending coverage: repeater, powerline, cable or mesh.
Cause 5: old firmware
An unpatched firmware carries memory management and routing table bugs that show up as a drop in performance over time. It’s the easiest cause to rule out and the most forgotten one.
Check: go into the router’s settings and look at the firmware version. Compare it with the one the manufacturer offers.
Fix: update, and only after making a backup of your current settings.
Cause 6: badly chosen 5 GHz channel
With 20, 40, 80 or 160 MHz channels in 5 GHz, a badly chosen channel overlaps with several neighbours at once. It’s the equivalent of your 2.4 GHz problem, but worse, because it overlaps more bands.
Check: look at the channel width your network uses on 5 GHz.
Fix: reduce the channel width to 40 MHz and let the router pick the least occupied channel.
Cause 7: problem at the internet connection
If the router is fine over Ethernet and bad over WiFi, but over Ethernet it still doesn’t reach the contracted speed, it isn’t a WiFi problem. It can be packet loss on the line, an overheating ONT or upstream saturation.
Check: test over Ethernet several times at different hours. If it always fails in the same window, that’s your ISP’s problem.
Fix: call technical support with the data from the Ethernet test.
Cause 8: slow DNS
This is the cause that gets blamed most and usually isn’t. DNS only affects how long it takes to load new pages, not your download speed.
Check: if downloads are fast but pages take a while to load, it’s probably DNS.
Fix: change to a fast DNS in the router, not on each device.
Order of action
| Symptom | Most likely cause |
|---|---|
| Slow all over the house, including over Ethernet | ISP problem |
| Slow only in one room | Coverage |
| Comes and goes, cuts out every so often | Interference or too many devices |
| Fast downloads, slow pages | DNS |
| Suddenly slow after weeks of being fine | Router firmware or memory |
If the cause becomes clear after measuring, go back to measuring your home WiFi signal and apply the corresponding change.
Quick checklist
- Ethernet test: your line’s ceiling.
- 2.4 GHz channel on 1, 6 or 11.
- Both bands active with different names.
- Number of connected devices.
- Firmware updated.
- 5 GHz channel width reduced to 40 MHz.
- DNS configured on the router.
External sources
- Wi-Fi Alliance: specifications — Wi-Fi 4/5/6/6E/7, security, EasyMesh, HaLow.
- IEEE 802.11: standards — PHY, MAC, channels, widths, modulation, QoS, security.
- IEEE 802.11ax: Wi-Fi 6 — OFDMA, MU-MIMO, TWT, BSS Coloring, coverage, efficiency.
- IEEE 802.11be: Wi-Fi 7 — 320 MHz, MLO, 4K-QAM, latency, multilink.
- ITU-R: spectrum recommendations — bands, channels, power, interference, coexistence.
- FCC: spectrum rules — channels, power, DFS, TPC, radar, coexistence.
- OpenWrt: WiFi configuration — channel, width, power, DFS, country, VAP, mesh.
- OpenWrt: WiFi optimization — channels, width, RTS/CTS, ACK timeout, RSSI, roaming.
- Linux Wireless: documentation — mac80211, cfg80211, regdb, DFS, TPC, scanning, authentication.
- Cisco: WiFi design guide — coverage, capacity, interference, RF, planning, site.
Next step
If the slowness comes with dropouts, the article you want is WiFi keeps disconnecting. If your problem is smart home devices, start with IoT devices disconnecting from WiFi, which has causes specific to them that don’t apply to a laptop.