Why would my download speed drop from 400 Mbps to around 4–20?

First figure out whether the slowdown is on Wi-Fi or the internet connection itself. When it happens, test from a computer plugged directly into the router with Ethernet, and compare that with Wi-Fi. If only Wi-Fi is slow, look at signal strength, interference, or router placement. If the wired test drops too, the problem is more likely beyond the wireless connection.

Check whether anything else is using the connection during those windows. Cloud backups, game downloads, system updates, or another device streaming can eat bandwidth. Test with one device connected, pause background downloads, and note whether the slowdown still happens. Also use the same speed-test service and server each time so the results are easier to compare.

If a wired test also shows the drop, keep a simple log with the time, result, and whether other devices were active, then contact your provider. Ask them to check for line problems or congestion and review the modem connection during the affected periods. The cause can depend on the connection type and local conditions, so the pattern alone is not enough to pin it on a specific fault.