Everything in the chain says gigabit on the box, but Windows reports a link speed of 100 Mbps and file copies crawl. Gigabit is fragile in one specific way: it needs all four pairs in the cable. Lose one, and the link politely falls back to 100.
First confirm the fact, not the feeling: open Settings, Network and internet, Ethernet (or the adapter status in Control Panel) and read Link speed. If it says 100 Mbps while both devices claim gigabit, this page is for you. And one honest note before the fixes: a 100 Mbps link is a ceiling, not a fault by itself - if your internet plan is 50 Mbps, you will never see the difference online. It matters for local file transfers, backups, and plans above 100 Mbps.
100 Mbps Ethernet uses two pairs of wires; gigabit uses all four pairs at once. So a cable with one damaged, loose or badly crimped pair often cannot do gigabit but works 'fine' at 100 - which is why this fault hides so well. The same applies to wall sockets and patch panels: one weak punch-down contact in the building wiring caps the whole run at 100 even with a perfect patch cable at your desk.
Sometimes the cap is not physical at all. If anyone previously fought a flapping link on this PC - a very common office story - they may have fixed Speed & Duplex at 100 Mbps or even 10 Mbps in the adapter's Advanced tab, exactly as shown in the main blinking-fix video, and the setting stayed. Open Device Manager, adapter Properties, Advanced tab: if Speed & Duplex is not on Auto Negotiation, try Auto on a known-good cable. If the link then holds at gigabit, you have found your cap. Also confirm the switch port itself is not administratively limited to 100 on a managed switch.
A gigabit PC into a gigabit router still lands at 100 if anything between them is 100-rated: an old small switch under the desk, a powerline adapter pair, a cheap network splitter that shares one cable between two ports (those force 100 by design), or an ancient patch panel. Walk the whole path from PC to router and read the rating printed on every box in between. The slowest box wins - always.
See the adapter settings changed live, step by step, in the full Technical Anshu video.
Watch Full VideoYes. Your connection to the router is capped at 100 Mbps, so nothing - internet or local - can pass faster than that through this cable, whatever your plan says. Fix the link to gigabit using the steps above and the full plan speed becomes reachable, assuming the rest of the path is gigabit too.
Original Cat5 was not rated for it, though very short perfect runs sometimes negotiate gigabit anyway - do not rely on it. Cat5e and Cat6 are the honest minimum for gigabit. When in doubt, the cable swap test answers faster than any label debate.
Yes. The link speed is the ceiling, not a promise. Slow copies at gigabit link usually mean a slow disk at one end, a busy PC, or Wi-Fi somewhere in the path. That is a throughput problem, not a negotiation problem - this page fixed the ceiling.