The short answer
Run a speed test on the slow device and on a fast one standing in the same spot. If the numbers differ hugely, the problem is that device: its Wi-Fi adapter is stuck on 2.4 GHz, its driver is out of date, or a background process is saturating the connection. Start with which band it has joined.
Other devices being fine is the most useful fact you have. It eliminates your internet connection, your router’s health and your provider in one step. Everything below is about the one device.
First, measure properly
Stand in one spot. Run a speed test on the slow device, then on a phone you know is fast, without moving. Same place, same moment. That controls for distance and interference, which otherwise explain away real differences.
Then check which band the slow device is on. If your router uses one name for both bands, the device may be clinging to 2.4 GHz while everything else enjoys 5 GHz. A device stuck on a congested 2.4 GHz channel can be five to ten times slower with no fault at all.
- Windows: Settings, Network and internet, Wi-Fi, then Hardware properties. It shows Network band.
- Mac: hold Option and click the Wi-Fi icon for channel and rate.
- Android: the network details usually show frequency.
- iPhone: not shown directly, so test by joining a separate 5 GHz network name.
The usual causes
Check what is using the connection
Open Task Manager on Windows, Performance, Wi-Fi, and watch throughput while you are doing nothing. On a Mac use Activity Monitor, Network. Cloud backup, Windows Update, a game launcher patching, or a sync client re-uploading a folder will saturate the link silently. This is the single most common cause of a device that is mysteriously slow for a day.
Force it onto 5 GHz
Split the bands into separate names in your router, then join the 5 GHz one directly. If the device only supports 2.4 GHz — common on older laptops and cheap tablets — that is your ceiling and no setting will raise it.
Update the Wi-Fi driver
On Windows, get the driver from the laptop maker’s support page for your exact model rather than Windows Update, which often installs a generic version with poor performance. This makes a larger difference than people expect, sometimes several times the throughput.
Turn off power saving on the adapter
Device Manager, Network adapters, your Wi-Fi adapter, Properties, Power Management, and untick the option to turn it off to save power. Then in Power Options, set Wireless Adapter Settings to Maximum Performance. A throttled adapter produces exactly this symptom.
Forget the network and rejoin
A stale profile can pin the device to a poor band or an outdated security mode. Forget, restart, rejoin.
Check for a VPN, proxy or DNS filter
A VPN routes everything through a distant server and can halve throughput. A DNS-based filter or a corporate proxy profile left over from an old job does the same. Disable each temporarily to test.
Test with the security software off
Some third-party antivirus suites inspect traffic in a way that heavily reduces throughput on certain hardware. Disable web shield briefly — not permanently — and retest to see whether it is responsible.
A laptop from 2015 with an 802.11n adapter tops out at a fraction of what a modern phone achieves on the same network. If the device is old, confirm what standard its adapter supports before troubleshooting further — a cheap USB Wi-Fi adapter is often a complete fix.
Frequently asked questions
Why is my laptop slower on Wi-Fi than my phone?
Usually the band or the adapter. Phones aggressively prefer 5 GHz and have newer radios, while laptops often sit on 2.4 GHz with a driver that has not been updated since the machine was sold.
Can one device slow down the whole network?
Yes. A device saturating the connection with a backup or a download consumes shared airtime, and on 2.4 GHz a slow device forces others to wait. If everything got slower at once, look for what one machine is uploading.
Does the number of bars mean anything?
Less than people think. Bars show signal strength, not speed or quality. A device can show full bars on a badly congested channel and still be slow, which is why a speed test on the device matters more.
Will a new router fix one slow device?
Rarely, if everything else is fast. The constraint is in the device. Spend the money on a Wi-Fi adapter or a driver update before replacing network hardware that is evidently working.
Test both devices in the same spot, then check the band. Those two steps identify the cause in most cases.
