Separate internet speed from Ethernet link speed
A speed test can be limited by the ISP, server, Wi-Fi, or device. First check the operating system or router interface for the negotiated Ethernet link rate; a 100 Mbps link points to the local wired path.
Swap the cable before changing settings
Gigabit Ethernet depends on all four twisted pairs. A damaged connector, pinched cable, poor termination, or marginal cable can still connect while falling back to 100 Mbps, so test with a short known-good Cat5e-or-better cable.
Check every port in the chain
The slowest port sets the ceiling. Confirm that the router LAN port, switch port, wall jack path, USB adapter, dock, and computer interface all support gigabit or faster speeds.
Leave auto-negotiation enabled unless diagnosing a specific fault
Manually forcing a speed can create a mismatch rather than fix the cable. Restore automatic speed and duplex settings, reconnect the cable, and confirm the negotiated rate at both ends.
Change one component at a time
Test device-to-router with one short cable, then add the switch, wall run, dock, or adapter back one by one. This turns a vague speed complaint into a specific failed cable, port, or intermediate device.