My PC’s internet speed suddenly dropped to around 100 Mbps

Try a different Ethernet cable and a different router or wall port if possible. Since your previous speeds were close to 1 Gbps and the new download speed is almost exactly 100 Mbps, a bad cable, connector, or port is more likely than your CPU, GPU, or storage.

Run the same speed test on another device connected to the same network, preferably a friend’s laptop. If every wired device is now slow, the problem is probably the router, cabling, or ISP connection. If only this PC is affected, compare its Ethernet link speed and adapter settings with the other device.

For Wi‑Fi, interference from a nearby network could cause a sudden drop. A Wi‑Fi analyzer can show crowded channels, and changing channels or using the 5 GHz band may help. Also check that the motherboard’s Wi‑Fi antennas are firmly connected and weren’t damaged or moved.

If Ethernet is also limited to around 100 Mbps, check the link speed in Windows. The adapter may have negotiated at 100 Mbps instead of 1 Gbps. Set Speed & Duplex back to Auto Negotiation, then restart the PC and router. A damaged cable or loose connection is also a common cause—Gigabit Ethernet needs all eight wires, while 100 Mbps can still work with only four.

Because both the wired and wireless connections seem affected, first confirm whether the speed test on another device is actually normal. If it is, focus on Windows network settings, adapter negotiation, the Ethernet cable, and the motherboard’s network hardware rather than changing DNS or reinstalling unrelated drivers.