My Windows 11 desktop suddenly started getting much slower internet about 2–3 days ago. Before this, I consistently saw roughly 900+ Mbps download and upload speeds, but now I'm getting around 90–100 Mbps down and only 30–40 Mbps up. Other devices on the same connection appear to work normally, and my phone gets proper speeds when using its hotspot.
I've already tried changing the IP and DNS settings, testing both Wi-Fi and Ethernet, restarting the network adapter, checking for LAN driver updates, updating the BIOS, running disk health checks and SFC, and restarting the computer. I also contacted my ISP, but they weren't able to identify a problem.
The PC has a Ryzen 7 7800X3D, ROG STRIX B650-A GAMING WIFI motherboard, Radeon RX 9070 XT, and 32 GB of DDR5 RAM. The hardware specifications seem unlikely to be relevant, but this slowdown appeared without any obvious change. What could be causing the connection to drop to roughly 100 Mbps, and what should I check next?
4 Answers
If the Ethernet connection is also stuck around 90–100 Mbps, check the negotiated link speed first. A gigabit connection should show 1.0 Gbps; if Windows reports 100 Mbps, the usual causes are a damaged cable, a loose connection, or a bad port. Gigabit Ethernet needs all eight wires in the cable, while 100 Mbps can still work with only four. Try a known-good Cat5e or better cable and another router or switch port.
Open the Ethernet adapter’s status or advanced properties and confirm the speed is set to Auto Negotiation rather than being forced to 100 Mbps. Rebooting both the computer and router can also make the devices negotiate the correct speed again. If it still negotiates at 100 Mbps with a different cable and port, the motherboard’s network port or the router port may be the issue.
Think about anything that changed shortly before this started, including moving the PC, opening the case, replacing cables, or a power event. A partially damaged cable, loose motherboard antenna, or damaged network port can produce exactly this kind of sudden slowdown. If every other device gets normal wired speeds and the PC remains limited after testing another cable and port, a USB Ethernet adapter can help confirm whether the built-in adapter is failing.
Since Wi-Fi can also be affected, compare the same speed test from another device while it is connected to the same network, preferably from the same location. If a friend’s laptop gets normal speeds but this PC does not, it points back to the computer. For Wi-Fi specifically, check the antenna connections and use a Wi-Fi analyzer to see whether interference or a crowded channel is causing problems.
That would be my first check too. A speed test near exactly 100 Mbps is often a sign that the physical Ethernet link fell back to Fast Ethernet, not that the ISP is randomly throttling the connection.