Resetting cross signing seems really heavy on federation

This issue has been migrated from [#7296](https://github.com/matrix-org/synapse/issues/7296).

---

So heavy that it breaks my small slow server for a while (1 shared vcpu and 2GB).
Nginx begins to fail because of too many opened connections/fds (ulimit is 524288).
It recovers after less than a minute once the federation spike is done.

However in the meantime the resetting fails on the client, probably because of a broken HTTP connection on the nginx. Clicking retry goes into the exact same behavior.

Once I also had to revalidate my unique device with the passphrase after doing a reset and it failed. Perhaps I was just too fast and the federation was just not over.