Your internet plan is faster than 100 Mbps, yet the computer reports a 100 Mbps Ethernet link. Start with the connection between the computer and its immediate network neighbour. Buying a faster plan will not change a local link that remains at the same rate.
Compare numbers that mean the same thing
Link speed is the negotiated connection rate; a browser speed test measures a transfer through a longer path. A download manager may use megabytes per second instead of megabits. Dividing 100 megabits by eight gives 12.5 megabytes per second before overhead, not a promised download result.
Write down both the reported link speed and the measured transfer speed, with units. A 1 Gbps link and a slow download call for a different investigation from a link that explicitly says 100 Mbps.
Draw the short path
List the pieces between the computer and router: adapter, dock, cable, wall outlet and any switch. Check the exact port specifications. Intel’s troubleshooting guidance stresses gigabit support at both ends and recommends auto-negotiation for its gigabit adapters. Forcing a speed setting is not a substitute for finding a limiting component.
Keep the first experiment small. Microsoft recommends checking the Ethernet cable and trying another one. Use a known-working cable, reconnect it firmly and note whether the reported link rate changes. If possible, test another suitable router port separately. Do not change cable, port and driver together: a success would leave you guessing what fixed it.
Keep the evidence before buying parts
For example, record “original cable: 100 Mbps; replacement cable on the same ports: 1 Gbps”. That is useful evidence about this path, though it does not identify the precise defect inside the old cable.
If the result remains unchanged, add the adapter and port model numbers to your notes and consult their manufacturer’s instructions. After any fix, repeat the transfer test. A higher link rate removes one possible bottleneck; it does not guarantee that the remote server or storage can keep up.