Large downloads start around 80 Mbps, but within a few minutes the speed gradually falls to 1–3 Mbps and stays there. This now makes downloading large games nearly impossible. Updating the Realtek PCIe GBE Family Controller drivers only delays the slowdown. The problem is not limited to one download service: after pausing a download, speed-test results remain around 1 Mbps until the modem is reset or the computer and modem are restarted. The computer uses an SSD, and a drive-health check reports that it is healthy. qBittorrent is installed, but no other devices were using the connection when this happened.
4 Answers
Because the connection also stays slow in speed tests after the download is paused, I would not assume this is only an SSD issue. That points toward the modem, router, Ethernet adapter, cable, or ISP connection. Try another Ethernet cable and router port, reboot the modem and router separately, check adapter power-saving and speed/duplex settings, and test with another computer if possible. If other devices are affected too, contact the ISP with the speed-test results.
A VPN can be a useful comparison test if the ISP is throttling a particular service, but it is less likely to help when ordinary speed tests also remain at 1 Mbps. Test the connection immediately after a reboot, then again once the slowdown appears, and record whether only this computer or the whole network is affected. That distinction will help separate a local SSD or network-adapter problem from a modem or ISP problem.
The SSD may be the bottleneck rather than its health status. Many lower-end SSDs use a small fast write cache; once that cache fills during a large download, sustained write speed can drop sharply. A drive can still report as healthy while performing poorly after its cache is exhausted. Check the SSD's free space, model, temperature, and sustained-write benchmark, and compare the result with the manufacturer's specifications.
Is freeing up space enough to fix that? This only started happening recently, even though I haven't changed anything obvious.
Make sure there isn't another process consuming bandwidth. Temporarily close qBittorrent and check its upload and download limits, then look at Task Manager's network usage or your router's connected-device list. A torrent client can continue transferring in the background even when the main download is paused, although that would not explain the issue if every device becomes slow after it is closed.
I tested while alone with the other devices turned off, and qBittorrent was not doing anything unusual, so it seems unlikely to be competing traffic.
The slowdown remains until I reset the modem or restart the PC, and it also happens when Steam is not downloading, so it doesn't seem exclusive to one application.