Fun with WiFi, again:

In my dataset of page load times over a connection that includes a WiFi link, I often observed outliers (unusually long load times).
At the client, sometimes there were just gaps in the trace, 600 ms during which no packet arrived. No retransmissions or anything though. No packets getting lost. Nothing wrong at the AP.
Finally I found data frames from the client saying that it goes into power save mode, even though I had turned power save off.

Guess what happened?
…

The client only said it went into power save, but actually went on other channels to perform a scan for other Access Points. During that time, of course it can't receive anything on the original channel.
Network Manager just scans every ~2 minutes, no matter if it's right in the middle of a download.
Well, it makes sense if you think there might appear other APs for the same SSID.
Solution: Set a fixed BSSID (= MAC address of an AP).
https://www.linux.com/learn/tips-and-tricks-get-most-out-your-linux-wifi