I jokingly posted it the other day.
100% a joke, there was no way it was DNS.
Today I found a configuration line.
It shouldn't matter, but just to exclude the impossible we had to try it.
It was frikkin DNS.
Not a network mesh latency issue, not a weird thing with the database, just some service (a shelf product, not our code) that incessantly looks up whatever names on every single request, without caching, and it was doing it from another continent, slowing everything to molasses.