Why does my download speed drop from 80 Mbps to 1–3 Mbps after a few minutes?

A VPN could be a useful diagnostic, but it is unlikely to be the real fix—especially since the slowdown continues outside the original download. Also pause qBittorrent completely and make sure it is not uploading heavily in the background. If restarting the modem consistently restores full speed, check its logs and firmware, and ask the ISP whether the connection is being throttled or the modem is overheating.

The fact that normal speed tests remain slow after pausing the download points more toward the network path, router, or Ethernet adapter than Steam itself. Try another Ethernet cable and router port, temporarily disable and re-enable the network adapter, and test with a different device on the same connection. Check whether the adapter is negotiating at 1 Gbps rather than 100 Mbps, and look for router or ISP traffic-shaping issues.

The explanation that the download simply fills your RAM is not quite right. Download clients can use memory and disk caching, but the sustained slowdown is more likely caused by the SSD reaching its cache limit, the drive overheating, or the network equipment/adapter becoming unstable. Monitor disk activity, SSD temperature, and Ethernet link speed at the moment the drop happens; that should identify which side is failing.

The SSD may still be the bottleneck even if its health status is fine. Many inexpensive SSDs have a small write cache; once a large download fills that cache, sustained write speed can drop sharply. Check the SSD model, available free space, and its write speed during a long file transfer. A nearly full or overheating SSD can make this worse, so leave some space free and monitor its temperature.