Why your internet is slow: a diagnosis in the right order

Slow internet is almost always one of four separate problems, and four questions separate them in about ten minutes. Is it one device or all of them? Does it happen on a cable as well as on wifi? Is it one service or everything? Is it all day or only at certain hours? Work through those in order and you will know whether to move a router, replace a laptop, change a setting or call your provider. Skip them, and you risk paying for a faster plan that fixes nothing, because a plan upgrade cannot help if the bottleneck is between the router and your hands.

Start with four questions

Run the questions in this order, because each one removes a whole category of cause. Do not jump ahead to the one you find most interesting.

You need real numbers to answer them, not impressions, so run the same test in each condition and write the results down. The method for that, including why a single result means very little, is in how to read a speed test properly.

One device or all of them

Test a second device in the same room at the same moment. If the phone is fast while the laptop crawls, the network is fine and you have a computer problem: a full disk, an update downloading in the background, too many browser tabs, a security scan, or a browser extension intercepting every page. Old machines also carry old wifi radios, and a laptop with a single stream 2.4 GHz radio will struggle to pass 50 to 70 Mbps however good your connection is. That ceiling belongs to the device, not the plan.

Check for a VPN too. A VPN encrypts your traffic and sends it through a distant server, which costs speed and latency by design, and people forget one is running far more often than you would expect. The trade offs are set out in what a VPN can and cannot do.

If every device is slow, move on. If only one is, the problem is that machine, and the practical cleanup is in why a computer gets slow and what actually fixes it.

Wifi problem or line problem

This is the split that matters most, and nearly everyone skips it. Plug a laptop into the router with an Ethernet cable, pause everything else on the network, and test. A wired test bypasses your wifi completely, so it measures what is arriving at your house.

Wired is good, wifi is bad: the line is fine and the fault is inside your home. Depending on where it fails, that is placement, the band your devices picked, or channel congestion.

Wired is bad too: the problem is the line, the router, or the cabling to it. Before blaming the provider, try the modem or router in a different socket, swap the Ethernet cable, and check for a cheap splitter or a loose, corroded connector on the incoming cable. Damaged coax and water in an old phone line are ordinary, boring causes of a genuinely slow connection.

One trap: a gigabit plan that always tests at about 94 Mbps on a cable is not a line fault. That is a 100 Mbps Ethernet port on the computer, the router or an old switch in between, hitting its physical ceiling.

One service or the whole internet

If a speed test flies but one site or app crawls, your connection is not the problem. Three causes cover most of it.

• The service itself. Video platforms, game download servers and cloud drives all have their own busy periods, and one region can be degraded while everything else works.
• The route between you and it. Traffic can take a congested path to one provider while every other destination is fine. Your speed test to a nearby server will never reveal this.
• Name lookup. If every site takes several seconds to start loading and then downloads at full speed, the delay is in translating names to addresses rather than in bandwidth. Switching to a different resolver can help, though it never adds speed to your line. How DNS decides where you land explains why the first few seconds behave differently from the rest.

Slow only at certain hours

An evening slowdown is the most common complaint and it has two completely different causes that look identical from the sofa.

Neighborhood congestion. Cable and some fixed wireless networks share capacity between homes on a local segment. When everyone streams at once, the shared portion fills up. The signature is that a wired test drops during peak hours and recovers late at night. There is nothing you can do about this from inside the house except report it with a log, because the fix is the provider adding capacity.

Airtime congestion on wifi. In apartments, dozens of networks share a handful of channels, and each transmission waits for the others to finish. The signature here is that wired speed stays perfect all evening while wifi collapses. That is fixable at no cost, by choosing a better channel or moving devices to a less crowded band, as covered in fixing wifi channel interference.

The distinction is the whole reason to keep a wired laptop plugged in while you diagnose. One of these is your provider's problem and one is yours.

The quiet causes people miss

Something is uploading. Photo backup, cloud sync, a security camera or a file sharing client can saturate a small upload channel, and a full upload channel slows downloads too, because the confirmations your computer sends back get stuck behind the outgoing flood. Pause sync services and retest. On an asymmetric cable plan this single cause explains a startling number of bad video calls.

An extender is halving your speed. A basic extender that receives and retransmits on the same radio can only use each slot once for listening and once for repeating, so devices behind it typically get around half the throughput, even with full signal bars. Choosing between that, a mesh system and a cable is covered in mesh, extender or cable.

Too many devices, or an old router. Every phone, speaker, doorbell and bulb takes a share of airtime even when idle. Routers supplied years ago with a service can struggle with a modern household, and heat makes it worse, so a unit crammed into a cupboard next to a set top box may be throttling itself.

The wrong band on the right router. A device that clings to 2.4 GHz while standing next to the router will be slow for reasons that have nothing to do with your plan. What the wifi bands trade against each other explains why devices make that choice and how to push them.

Placement. If the router sits in a hallway cupboard by the front door, behind a metal panel, the signal has already lost most of its strength before it reaches the rooms you use. Moving the router is free and beats most hardware purchases.

When to call, and what to say

Call when a wired test, with the network quiet, comes in well below your plan at several different times of day, or when packet loss appears repeatedly on a wired test. Those two findings are the provider's responsibility and are hard to argue with.

Bring a log of at least ten tests with dates, times and all five figures, state plainly that they were wired with other devices idle, and ask directly whether there is a known fault, what rate the line is synchronized at, and whether the local segment is congested. Ask for the case number. If the first call ends with a reset and no explanation, the log is what gets you to the second level.

Before any of that, confirm you are not simply outgrowing a small plan. Four people streaming and working at once on a 50 Mbps connection is not a fault, it is arithmetic, and how much speed a household really needs will tell you quickly which side of that line you are on.