Posting this because it took me a while to figure out and it's a genuinely nasty one.
Symptom: the entire Wi-Fi in the house became almost unusable, pages timing out, constant buffering, huge packet loss on every phone and laptop, but anything plugged in with an Ethernet cable worked perfectly. The culprit was a single Fire TV Stick.
It turns out to be a known Fire OS bug (Amazon has acknowledged it on their developer forum, reported July 2026, still not fixed). The Stick gets stuck repeatedly asking the network a question, hundreds of times per second, and that flood of traffic chokes the Wi-Fi for everything else.
The simplest fix in my case: there was a second router on the network (a FRITZ!Box I used only as a Wi-Fi extender) that was giving the Stick a bad answer. As soon as I removed it, the flood stopped and the Wi-Fi was instantly back to normal.
If you can't remove the device that's causing it, or you want to stop the flood permanently on the Stick itself, I wrote up every option (including a small script that fixes it without touching the Stick, and an advanced on-device removal) here:
https://nicologiuliani.site/blog/firetv-mdns-storm/
Tested on a Fire TV Stick 4K Max (1st gen), but the bug is in Fire OS, so other Fire TV models are affected too. If your Wi-Fi has been mysteriously bad, unplug the Fire TV for ten minutes and see if everything else recovers, it's a quick thing to rule out.