Why is cable internet speed slower than Wi-Fi: technical reasons and solutions

You connected your computer to the router via Ethernet cable, expecting maximum speed, but tests show results worse than expected Wi-FiThis situation seems paradoxical: after all, a wired connection has always been considered more stable and faster. However, in practice, users regularly encounter the opposite situation: The speed over a cable drops by 20-50% compared to a wireless network, and sometimes even falls to critical values.

In this article we will look at 7 Key Reasons, why this happens—from trivial errors in router settings to hidden hardware limitations. You'll learn how to properly diagnose the problem, which parameters to check first, and what to do if your ISP, not your PC, is to blame. We'll also cover myths (for example, about the "harmfulness" of Auto Negotiation) and little-known nuances that even experienced users ignore.

Spoiler: in 60% of cases, it’s not the cable that’s at fault, but Incorrect network card drivers or duplex mode, which the router sets automatically. But first things first.

1. Ethernet Cable Problems: From Standard to Physical Damage

The first thing that comes to mind when you have low speed over cable is problems with the wire itself. And this is logical: if Wi-Fi If it works faster, then the "weak link" in the chain is to blame. However, things aren't so clear-cut. Let's figure out which cables can actually limit speed, and which can't.

Standard Cat 5e (the most common) supports speeds up to 1 Gbps at a distance of up to 100 meters. But there are two nuances here:

• 🔌 Quality of braiding and connectors: cheap cables with thin or poorly crimped wires RJ-45 may create interference, reducing the actual speed to 100 Mbps (or even less).
• 📏 Cable length: when the distance exceeds 50–70 meters, the signal begins to fade, especially if the cable is laid near sources of electromagnetic interference (for example, near a refrigerator or microwave).
• 🔄 Bends and damages: even microcracks in the braid or pinched areas can lead to packet loss (packet loss), which manifests itself as speed "freezing".

How do you check? Connect the cable to another device (e.g., a laptop) or try replacing it with a known-good one. If the speed increases, the problem is with the cable. If not, keep looking.

⚠️ Attention: If you use extension cords or splitters For Ethernet (for example, to branch a single cable to several devices), they can become a bottleneck. Most cheap splitters limit the speed to 100 Mbps, even if the cable supports 1 Gbps.

2. Incorrect router settings: duplex mode and MTU

The router is the brain of your network, and its settings directly affect the speed of both Wi-Fi, and by cable. Two key options that are often confused or installed suboptimally:

1. Duplex Mode: May be Full Duplex (full duplex, simultaneous reception and transmission of data) or Half Duplex (Half-duplex, data is transmitted only in one direction at a time). If the router and the PC's network card fail to agree on the mode, the speed drops by 2-5 times.
2. MTU (Maximum Transmission Unit): The maximum data packet size. If this value is too high, packets are fragmented, increasing network load. If it is too low, traffic is transmitted less efficiently.

How to check the settings on your router:

1. Go to the router's web interface (usually at 192.168.0.1 or 192.168.1.1).
2. Find the section LAN or Local area network.
3. Check the parameters Speed & Duplex (speed and duplex). The optimal value is Auto (auto detection) or 1000 Mbps Full Duplex (if your network is gigabit).
4. For MTU the default value is 1500If you have PPPoE-connection, try reducing it to 1472.

3. Drivers and settings for the network card on the PC

One of the most common reasons for cable speed drop is outdated or incorrect network card driversThis is especially true for built-in network adapters on motherboards (for example, Realtek RTL8111 or Intel I219-V).

How to check and update drivers:

1. Open device Manager (Win + X → Device Manager).
2. Expand the branch Network adapters.
3. Find your network card (eg. Realtek PCIe GbE Family Controller).
4. Right click → Update driver → Search for drivers on this computer → Select a driver from the list....
5. If there are multiple versions available, select the newest one (usually marked Microsoft or release date).

Additional network card settings worth checking:

• 🔧 Disabling energy saving: in the adapter properties (Power management) uncheck Allow the computer to turn off this device to save power.
• 📥 Offloading parameters: tab Additionally → turn off IPv4 Checksum Offload, TCP Checksum Offload And Large Send Offload (sometimes they conflict with routers).
• 🔄 Speed ​​and duplex: if the router has Auto, there should be one here too Auto Negotiation.

⚠️ Attention: On some motherboards ASUS And Gigabyte integrated network cards Realtek have a bug with the driver version 10.045 and newer, due to which the speed is limited 100 MbpsThe solution is to roll back to the version 10.040 or use the driver from Microsoft.

4. Provider restrictions: tariffs and technical support

Sometimes the problem isn't with your hardware, but with provider settingsHere are three scenarios where he is to blame:

1. Cable tariff with restrictionsSome operators (especially small local providers) artificially limit the speed Ethernetto relieve the network. For example, Wi-Fi you get 300 Mbps, and by cable - only 100 Mbps.

2. MAC address bindingIf your provider has linked your tariff to Router MAC addressIf you connect your PC directly (bypassing the router), the speed may be limited to a minimum. The solution is to clone the router's MAC address to the PC's network card or call support.

3. Problems on the lineThe cable from the provider to your apartment is damaged, faulty ONU/ONT terminal (for fiber optics) or an overloaded switch in the entrance hall - all of this can manifest itself as low speed only over a wired connection.

How to diagnose:

1. Connect the cable directly to your PC (bypassing the router) and check the speed. If it increases, the problem is with the router. If not, call your ISP.
2. Call support and ask if there are any restrictions Ethernet for your plan. Check if it's linked to a MAC address.
3. Ask to check signal level (for fiber optic) or attenuation (for twisted pair). Normal values:
   • 🌐 For GPON: the signal level must be in the range -20...-28 dBm.
   • 📶 For ADSL/VDSL: attenuation no more than 30–40 dB.

What should I reply to technical support?

Please clarify whether there is a speed difference between Wi-Fi and Ethernet for my plan. Tell them that the speed is lower when connecting directly to the PC than through the router. Ask them to check the MAC address binding and the signal strength on the line.

5. Router CPU overload and hardware limitations

If your router cheap or outdated, it may simply not be able to handle the load when running simultaneously Wi-Fi And EthernetThis is especially noticeable on models with a single-core processor (for example, TP-Link TL-WR840N or D-Link DIR-300).

How does this manifest itself:

• 📉 The cable speed drops during active use Wi-Fi (for example, when someone watches a 4K video).
• 🔥 The router gets very hot and may reboot spontaneously.
• 🕒 Delays (ping) increase to 100–300 ms even on a local network.

Solutions:

• 🔄 Disable unnecessary features: QoS, Parental Control, IPv6 (if you don't use it).
• 🔧 Update the firmware router to the latest version (sometimes manufacturers optimize traffic processing).
• 💻 Divide the networks: if the router supports VLAN, select Ethernet And Wi-Fi into different subnets.
• 🛒 Buy a new router with a dual-core processor (eg ASUS RT-AX55 or Keenetic Hero).

⚠️ Attention: Cheap routers with Realtek RTL8196C or MediaTek MT7620 often have a bug where the speed Ethernet falls to 10 Mbps under high load. The only solution is to replace the device.

6. Viruses, background processes, and Windows limitations

If the cable speed is low only on one PC, and on other devices (for example, Smart TV or laptop) everything is fine - the problem is systemHere's what might be interfering:

1. Viruses and mining botsSome malware uses the network card for covert cryptocurrency mining or DDoS attacks, which leads to bandwidth overload.

2. Background updates. Windows Update, Steam, Epic Games or NVIDIA GeForce Experience can download gigabytes of data in the background.

3. Group Policy LimitationsIn corporate or educational networks, administrators may artificially limit the speed of Ethernet through gpedit.msc.

How to check:

1. Open Task Manager (Ctrl + Shift + Esc) → tab Performance → EthernetSee how much traffic the system is consuming.
2. Run your antivirus (for example, Kaspersky Virus Removal Tool) and check the system for mining bots.
3. Disable background updates:

   net stop wuauserv
   net stop bits

   (this will stop temporarily Windows Update).

4. Check the group policy:

   gpedit.msc → Computer Configuration → Administrative Templates → Network → QoS Packet Scheduler

   Make sure the parameter Limit reserved bandwidth disabled.

7. Port problems: from the router to the network card

Last but not least, faulty ports on the router or network card. Even if the cable is intact and the settings are correct, the physical port may not be working correctly.

How to diagnose:

• 🔌 Try a different port on your router.For example, if you are connected to a port LAN1, switch to LAN2.
• 💻 Connect the cable to another device (laptop, Smart TV, game console). If the speed is normal, the problem is with the PC's network card.
• 🔍 Look at the port indicators:
  • 🟢 Green/yellow light - connection on 1 Gbps.
  • 🟠 Orange - connection on 100 Mbps or lower.
  • 🔴 Flashing red - data transmission errors.

If the port on the router is faulty:

• Try resetting the router to factory settings (button Reset for 10 seconds).
• Update the firmware.
• If that doesn't help, the port is burnt out, and the router needs to be repaired or replaced.

If the problem is in the PC network card port:

• Try using USB-Ethernet adapter (For example, TP-Link UE300).
• If you have PCI-E network card, move it to another slot.
• On laptops, sometimes disabling the built-in network card helps. BIOS and using an external adapter.

FAQ: Frequently Asked Questions About Ethernet vs. Wi-Fi Speed

Why is the speed via Wi-Fi 300 Mbps, but via cable only 100 Mbps?

This is a typical situation for routers with Fast Ethernet ports (limited 100 Mbps). Check your router's specifications: if the ports are marked as 10/100 Mbps, they physically cannot provide a higher speed 100 MbpsThe solution is to buy a router with Gigabit Ethernet (10/100/1000 Mbps).

Can a virus only reduce speed via cable and leave Wi-Fi alone?

Yes, some viruses (eg. mining bots or Trojan Proxy) can be used purposefully wired connection, so as not to arouse suspicion in the user (since Wi-Fi usually controlled more strictly). Check the processes in Task Manager and scan the system Malwarebytes or Dr.Web CureIt.

Why does cable speed drop at night?

Most likely, this is due to:

1. Overload of the provider's network (at night many people download torrents or updates).
2. Background updates on your PC (Windows, games, drivers).
3. Tariff restrictions (Some providers throttle speed during peak hours).

Solution: Run the speed test on another device (for example, a smartphone) Wi-Fi). If the same problem occurs there, the provider is to blame.

How can I check if my router is limiting my cable speed?

Connect the cable directly to your PC (bypassing the router) and run a speed test. If the speed increases, the router is indeed throttling your traffic. Possible causes:

• Incorrect settings QoS (traffic prioritization).
• Router processor overload.
• MAC address restriction (if the router "does not recognize" your PC).

Solution: reset the router to factory settings or update the firmware.

Can old Cat 5 cable limit speed to 100 Mbps?

Theoretically Cat 5 supports up to 100 Mbps, A Cat 5e — to 1 GbpsHowever, in practice, even Cat 5e can work on 100 Mbps, If:

• The cable is damaged or has bad connectors.
• The length exceeds 50 meters.
• The router or network card does not support 1 Gbps.

Check the cable with a tester or replace it with a new one. Cat 6.