A faster internet plan won't feel faster if your PC is silently capping itself at 100Mbps. That's the trap one MakeUseOf writer fell into after upgrading to a 150Mbps plan and seeing speed tests return barely one-third of what they paid for.
The culprit wasn't the ISP, wasn't the router, wasn't the plan. It was a Windows status screen most people never open, showing the Ethernet link speed between the PC and the router. Once that was found and fixed, full speed returned.
Why link speed is your hidden speed limit
Most people only check their plan speed: 50Mbps, 150Mbps, 1Gbps. That's the rate flowing in from your ISP. Link speed is different. It's the negotiated rate between your PC and the router over the Ethernet cable, and it operates completely independently of your internet plan.
Common link speeds are 100Mbps, 1Gbps (Gigabit), and 2.5Gbps on newer hardware. Your PC and router perform a quick handshake called Auto-Negotiation and pick the highest speed both devices support. Cable matters here—Cat5 tops out at 100Mbps; Cat5e and Cat6 handle Gigabit; Cat6a and Cat7 go higher.
Quick cable reference
| Cable Type | Max Speed | Gigabit Compatible |
|---|---|---|
| Cat5 | 100Mbps | No |
| Cat5e | 1Gbps | Yes |
| Cat6 | 1Gbps (10Gbps up to 55m) | Yes |
| Cat6a | 10Gbps | Yes |
Check link speed in seconds: Open Control Panel → Network and Internet → Network and Sharing Center → Change adapter settings. Right-click your Ethernet adapter, choose Status. The "Speed" line shows the negotiated rate.
Why this matters more than you think
The real danger is invisibility. The internet still works. Pages load. Speed tests return a number. That number just isn't the one your plan promises, and most people blame the ISP first. They call support, get cleared, and live for months with a slower connection than they pay for.
This is wasted money and a silent performance hit for anything bandwidth-intensive—game downloads, large file transfers, streaming 4K content, or moving files to a NAS.
The Ethernet cable is the most overlooked culprit. Physical trauma (office chair rolls, door pinches, age) damages the internal twisted pairs. Gigabit Ethernet needs all four pairs working; 100Mbps only needs two. Damage one pair, and Auto-Negotiation steps the whole link down without breaking the connection.
Who should care most:
- Remote workers on video calls who blame Wi-Fi for stuttering
- Gamers updating large titles and wondering why downloads crawl
- Anyone who recently upgraded their plan but didn't see the speed increase
- Users with older cables run behind furniture or through walls
Troubleshooting checklist
Step 1:** Check link speed under Network and Sharing Center. If it shows 100Mbps on a Gigabit-capable cable, something is wrong.
Step 2:** Open Device Manager → Network adapters → right-click your Ethernet adapter → Properties → Advanced tab. Find "Speed & Duplex." Confirm it is set to Auto Negotiation, not a manually pinned lower speed.
Step 3:** Swap the cable. Use a known-good Cat5e or Cat6 directly from PC to router. Bypass wall sockets and switches if possible.
Step 4:** Try a different port on the router. Recheck the link speed. If it jumps to 1Gbps, the old cable or wall run is faulty.
This sequence takes about ten minutes and catches nearly every common cause.
What to watch next
Windows sometimes resets adapter settings after major updates, so recheck "Speed & Duplex" if speeds mysteriously drop post-update. Motherboard-provided drivers often handle Auto-Negotiation more reliably than Windows Update defaults.
For future-proofing, Cat6 is worth the small price premium over Cat5e. Its higher 250MHz bandwidth leaves headroom for 10Gbps transfers to NAS devices, and its better crosstalk protection keeps signals clean when multiple cables run side by side—common in home office setups.
Bottom line: if your speed test can't pass your plan's number, look at the link speed first. A cable swap beats an hour on hold with your ISP.
Key takeaways
- Link speed and internet plan speed are separate; one can bottleneck the other.
- Windows shows the negotiated link speed in the Network and Sharing Center adapter status.
- A damaged cable forces Auto-Negotiation down to 100Mbps while the connection stays alive.
- Always confirm Speed & Duplex is set to Auto Negotiation before swapping cables.
Frequently asked questions
How do I find my Ethernet link speed in Windows?
Open Control Panel → Network and Internet → Network and Sharing Center. Click "Change adapter settings," right-click your Ethernet adapter, choose Status, and read the Speed line.
Why is my link speed 100Mbps when my cable is Cat5e?
Cat5e supports Gigabit. A 100Mbps link usually points to a damaged cable, a bad port, or a manually pinned "Speed & Duplex" setting. Swap the cable first.
Can a bad Ethernet cable cause slow internet without disconnecting?
Yes. Internal wire damage forces Auto-Negotiation to fall back to 100Mbps while the connection stays up, silently throttling everything.
Does Wi-Fi have a similar link speed setting?
Wi-Fi shows a connection speed in the same adapter Status window, but its troubleshooting path differs. Restart the router, update drivers, and check for interference first.
Will a Cat6 cable fix slow speeds?
Only if your current cable is the bottleneck and your network gear supports Gigabit or faster. Otherwise, the cable change won't help.