My other devices get normal internet speeds, and my PC is connected by Ethernet. Earlier today, a speed test showed around 655 Mbps, and the first download worked normally. Shortly afterward, download speeds dropped to about 0.16 Mbps or 200 KBps. Even when I stop all downloads and wait, the speed test briefly shows around 600 Mbps before falling to 30 Mbps or less. What could cause this, and how can I troubleshoot it?
4 Answers
Since your speed test itself drops after initially showing a high result, the problem may be local rather than just the download server. Restart the PC and router, then test with another Ethernet cable or router port. Updating or reinstalling the Ethernet adapter driver and checking its negotiated link speed can also help identify a faulty cable, adapter, or port.
A speed test only measures the connection to that particular test server. The server hosting your download, or the network path between you and it, may be slow or overloaded. Try downloading from a few different services and compare the results.
If the connection repeatedly jumps to 600 Mbps and then collapses while the computer is idle, watch Task Manager’s Ethernet graph during the drop. If network usage stays low but the test reports poor speed, try disabling power-saving features for the Ethernet adapter and temporarily testing with Wi-Fi or another device on the same cable and port.
First check Task Manager for both network and disk activity. Another program could be using the connection or maxing out the drive receiving the download. Also check for cloud-sync apps, game launchers, Windows updates, and antivirus scans running in the background.

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