Most likely something on that channel is spamming multicast. That kills most consumer wifi routers (in default settings). Usually something like a sonos or Google home broadcast group.
I might (i.e. I definitely do) have a non-ideal setup at home that contributes to this, with the router/WPA, a RPi running HA with a Zigbee antenna just next to it, my server in the shelf next over and a Sonos above it. Worst of all, the server is running on WiFi and it is sat in immediate vicinity to my router. Why? Because I could not for the life of me make the ethernet transfer speeds be more than somewhere around 1-5 MiB/s, so I gave up. But considering these issues are so infrequent from this location I assume it is mostly due to outside interference.
And adding to that, ....
Use an app like PingTools (Android) that can graph what is on each wifi channel. Check to find the cleanest channels in your area and configure your router to use that channel.
... this scan shows that all discovered networks are occupying the same channels (98-114 it looks like). I have still not figured out what OpenWRT option to go for (the OpenWRT One, which I had originally planned, quickly became very expensive with imports and tolls that I ended up not buying, despite having decided to go for it). So I am using the stock ISP router, which I assume everyone else in my building are as well. And I have been unable to locate an option to change channels in its interface.
If you’re curious and technically-minded, I highly recommend this write up: https://www.wiisfi.com/
That is a resource I didn't know I needed! Thanks :)
Among other reasons, a well-established, decentralized mesh network builds community resilience that can be very useful in emergencies where centralized infrastructure is compromised.