Rebooting my router did not fix my DNS problems. I have discovered the cause of the problem, however.

Please write in your guesses. First correct answer wins a prize.

Update: we have a winner https://mendeddrum.org/@fanf/116467456344983743 - it was dnsmasq calling fsync(2) every time it writes the dhcp leases file on my network with ~80 devices, thus making the process unresponsive until the write was committed. I fixed it with libeatmydata.

Thank you all for playing.

@jackeric arguably not