Short answer: leave your network adapter’s speed setting on auto-negotiation, then suspect the cable. Intel’s support guidance for its own adapters says that if the setting is right and the link still reads 100 Mbps, the problem is almost always the Ethernet cable.
If your speed is slow but the link already reads 1 Gbps, the bottleneck is somewhere else: see not getting the speed you pay for.
Where this advice comes from
The steps below come from Intel’s support article on links capped at 100 Mbps. It’s written for Intel’s Killer network adapters on Windows and was last reviewed in 2021, so menu names may differ on other adapters and systems.
Check the link speed
On Windows, Intel says to check it under Control Panel > Network and Internet > View network status and tasks.
Leave the adapter on auto-negotiation
Intel’s guidance is that the only setting that matters for a gigabit connection is that the adapter’s Speed & Duplex is set to Auto Negotiation, which is the default, in Device Manager under the adapter’s Advanced tab. If it has been changed, set it back to Auto Negotiation.
Find the weak link
Intel lists these steps: reseat the cables, swap in a single proven Cat 6 cable, connect the PC directly to the modem, power-cycle the switch or router, and try different ports.
Planning faster wired connections? Multi-gig Ethernet explains what every link needs for speeds above 1 Gbps.