Why Is My Internet So Slow? 10 Causes and How to Narrow Them Down
When your internet feels slow, the line itself is not always the problem. The cause is often inside your home: the Wi-Fi signal, the router, a network cable, or settings on your computer.
In this article: how to measure first and confirm that your connection really is slow, a step-by-step way to narrow down the cause by changing one thing at a time (wired vs. Wi-Fi, time of day, device), and how to spot and fix ten common causes.
First, measure to confirm it really is slow
If you go by feel alone, it is easy to blame the wrong thing. Start by running our speed test to measure download, upload and ping, then compare the results with the guide below. Ping is the round-trip time: how long it takes for data to reach a server and for the reply to come back.
| Measure | Too slow / poor | Standard / good | Plenty / very good |
|---|---|---|---|
| Download | Under 10 Mbps (HD video may stall) | 10–30 Mbps (HD video and video calls work; 4K may be unstable) | 30–100 Mbps is enough (4K, several devices at once); 100 Mbps or more is fast |
| Upload | Under 5 Mbps (camera in video calls and streaming will struggle) | 5–10 Mbps | 10 Mbps or more |
| Ping | 50 ms or more (a disadvantage in online multiplayer games) | 20–50 ms | Under 20 ms |
If your numbers meet the guide but things still feel slow, the website or app you are using may be congested on its end. If your numbers fall below the guide, move on to the steps in the next section. We explain these thresholds in more detail in what internet speed you need.
Step-by-step: narrowing down the cause
Change one condition at a time and measure again. Comparing the results shows you where the problem is. Work through the steps in order.
| Step | What to do | How to read the result |
|---|---|---|
| 1 | Measure in your current setup and compare with the guide | If the numbers meet the guide, suspect something other than your connection (such as the website) |
| 2 | Connect a computer to the router with a network (Ethernet) cable and measure | If only wired is fast, Wi-Fi is the cause. If wired is slow too, go to step 3 |
| 3 | Measure in the morning or midday, and again in the evening between 20:00 and 23:00 | If only evenings are slow, congestion at your provider or inside your building is likely |
| 4 | Measure on a different phone or computer | If only one device is slow, that device is the cause |
| 5 | Restart the router and the ONU, then measure | If speed improves, the equipment had a temporary fault |
An ONU (optical network unit) is the box that converts the signal from the fiber line into a signal your home network can use. While testing, pause video and downloads on other devices. Run the test two or three times and look at the average; that makes the results easier to compare.
Causes inside your home (1–5)
1. Wi-Fi signal (distance, walls, a crowded 2.4GHz band, old standards)
How to spot it: wired is fast, but Wi-Fi is slow. Speed drops the farther you are from the router, and when walls or floors are in the way. The 2.4GHz band is easily crowded by microwave ovens and neighbors' Wi-Fi networks.
What to do: place the router in a high, central spot in your home, and connect to the 5GHz band (or the 6GHz band). If your router or device still uses an old standard, the limit of that standard holds you back. The theoretical maximum for each standard is listed below. Real-world speeds are much lower.
| Standard | IEEE name | Bands | Theoretical maximum |
|---|---|---|---|
| Wi-Fi 4 | 802.11n | 2.4GHz / 5GHz | 600 Mbps (5GHz, 40MHz channel, 4x4 product spec) |
| Wi-Fi 5 | 802.11ac | 5GHz | About 7,000 Mbps |
| Wi-Fi 6 | 802.11ax | 2.4GHz / 5GHz | About 9,600 Mbps |
| Wi-Fi 6E | 802.11ax | 2.4GHz / 5GHz / 6GHz | About 9,600 Mbps |
| Wi-Fi 7 | 802.11be | 2.4GHz / 5GHz / 6GHz | About 36,000 Mbps (Intel figure; theoretical value based on the draft standard) |
How to measure Wi-Fi and what to expect is covered in our Wi-Fi speed guide.
2. Router or ONU problems
How to spot it: it used to be fast and suddenly got slow. A light on the device is a different color or blinking differently from usual. It has been powered on for a very long time.
What to do: turn off the ONU, then the router. Wait a few minutes, then turn the ONU back on first, followed by the router. If that does not help, update the router's firmware (the software that controls the device).
3. Network cable category
How to spot it: even on a wired connection, speed tops out at around 100 Mbps. Check the category printed on the cable jacket, such as "CAT5".
| Category | Maximum speed | Bandwidth |
|---|---|---|
| CAT5 | 100 Mbps | 100MHz |
| CAT5e | 1,000 Mbps | 100MHz |
| CAT6 | 1,000 Mbps | 250MHz |
| CAT6A | 10,000 Mbps | 500MHz |
| CAT7 | 10,000 Mbps | 600MHz |
| CAT8 | 40,000 Mbps | 2,000MHz |
What to do: replace CAT5 cables with CAT5e or better. The same ceiling also appears when the network port on your router or computer only supports up to 100 Mbps, so check the equipment specs as well.
4. Your computer (background updates, security software, an old Wi-Fi adapter)
How to spot it: only that computer is slow, while your phone is fast. Operating system updates or cloud sync may be running in the background.
What to do: wait for updates to finish, then measure again. Temporarily turn off the network scanning feature of your security software and see whether the result changes. If the difference is large, review its settings. Replacing an old USB Wi-Fi adapter with one that supports a newer standard can also help.
5. Number of devices connected at once
How to spot it: it is slow only when other people in the household are streaming video or gaming. All connected devices share the speed of your line.
What to do: schedule large downloads and game updates for times when nobody is using the connection. In homes with many devices, a router that supports a newer standard and handles many simultaneous connections better is also an option.
Causes on the line or plan side (6–7)
6. Wiring inside apartment buildings (VDSL or LAN wiring)
How to spot it: you live in an apartment building, and even a wired connection never goes above 100 Mbps. In some buildings, fiber reaches only a shared equipment room, and the last stretch to each unit runs over existing telephone wiring (VDSL) or a building LAN. If the wall outlet in your unit is a telephone jack, VDSL is a possibility. As an example, in Japan NTT East states that the apartment VDSL/LAN wiring types of its FLET'S Hikari Next service have a maximum speed of roughly 100Mbps for both download and upload. That is a maximum; actual speed can be lower depending on the building's equipment and how busy it is.
What to do: ask the building manager how the building is wired, and whether fiber can be run all the way to your unit. According to NTT East, in buildings that only have VDSL/LAN equipment, fiber equipment has to be installed in a shared area with the building manager's approval.
7. Provider congestion (PPPoE vs. IPoE)
How to spot it: speed drops sharply only in the evening between 20:00 and 23:00, and ping gets worse too. Tests at midday are fast.
PPPoE is an older connection method where you log in with a user ID and password. NTT East explains that PPPoE traffic passes through network termination equipment (a BRAS), which tends to get congested when many people are online, and speed can drop as a result. IPoE does not go through a BRAS; it passes through the gateway routers of a VNE (virtual network enabler, a carrier that provides the IPv6 network). IPoE supports IPv6 natively, and IPv4 can be used over it with methods such as "IPv4 over IPv6". These terms come from Japan's fiber network, but the general point applies anywhere: the way your provider connects you to the internet can affect your speed at peak hours.
What to do: check whether your provider offers a connection method that avoids this bottleneck (in Japan, IPoE), and whether your router is actually using it. If it is supported, a request to the provider or a change to router settings may be all it takes.
Other causes (8–10)
8. Data caps (mobile connections)
How to spot it: your phone or mobile hotspot suddenly got slow partway through the month. On many plans, speed is throttled once you use up your monthly data allowance.
What to do: check your remaining data in your carrier's app or account page. Wait until the next billing cycle, buy extra data, or switch to Wi-Fi.
9. DNS
DNS is the system that translates website names into IP addresses. How to spot it: your speed test results are fine, but there is a delay before pages start to load.
What to do: restart the router. If that does not help, review the DNS settings on your computer or router. If you change them, write down the original settings first.
10. Malware
How to spot it: there is a lot of network traffic even when you are not doing anything. Only one device is slow, and you see software or ads you do not recognize.
What to do: update your operating system and security software, then run a full scan. Disconnect any suspicious device from the network, and change your passwords from a different device.
When to consider changing your plan or provider
If your connection is slow only in the evening and stays that way for weeks, or if it never reaches the speed you need even on a wired connection, changing your plan or provider may be worth considering. Before you do, use the steps above to confirm that the cause is not inside your home. That way you avoid paying for a change that does not help. Also keep in mind that if your building uses VDSL wiring, switching providers will not remove the building's own speed ceiling.
When comparing options, first check the connection technology each one uses to reach your home (fiber all the way, VDSL, cable, fixed wireless or mobile), since that sets the upper limit. Ask how the service performs at peak hours rather than relying only on the advertised maximum. Look for a trial period or a short cancellation window, so you can measure it yourself before committing. Prices and coverage change, so confirm the details on each provider's official pages before signing up. For ways to get more out of your current connection, see how to improve your internet speed.
FAQ
- Why did my internet suddenly get slow?
Common causes are a temporary fault in the router or ONU, updates running on your computer, or, on mobile, a data cap. Restart the router and the ONU first, then measure again. If that does not help, compare a wired test with a Wi-Fi test.
- Why is my internet slow only at night?
Your provider's equipment or the line inside your building may be congested at the hours when most people are online. PPPoE connections are said to be more affected by this congestion. Check whether your provider offers a connection method that avoids it, such as IPoE.
- What should I do if only my Wi-Fi is slow?
Measure again close to the router, connected to the 5GHz band. If it is fast up close, distance or walls are the cause. Move the router, or check the Wi-Fi standards your device and router support.
- Why does my wired connection top out at around 100 Mbps?
Likely causes are a CAT5 network cable, a network port that only supports up to 100 Mbps, or VDSL/LAN wiring in an apartment building. Check the printing on the cable, the specs of your equipment, and the type of outlet in your wall.
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- GuidelinesWhy Is My Wi-Fi Slow? Bands, Standards and What to Fix First
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Sources
- NTT East, "What is IPoE? Comparing PPPoE, IPv6 and IPv4" (Japanese; accessed 2026-11-12)
- NTT East, "Monthly fee revisions for some services and end of discounts" (Japanese; accessed 2026-11-12)
- Sanwa Supply, "How to choose a LAN cable" (Japanese; accessed 2026-11-12)
- Sanwa Supply, "How to tell LAN cable categories apart" (Japanese; accessed 2026-11-12)
- Intel, "Wi-Fi generations at a glance" (accessed 2026-11-12)
- Buffalo WSR-3200AX4B product specifications (Japanese; accessed 2026-11-12)