SSH takes ten seconds to reach a prompt

The connection opens quickly but the password prompt takes ten seconds. The server log mentions reverse mapping checking getaddrinfo.

sshd is resolving the client address backwards, and DNS is slow or unreachable:

UseDNS no
GSSAPIAuthentication no

The second line matters too. Many distributions enable GSSAPI by default; without a Kerberos ticket the client tries to negotiate and only falls back after a timeout.

systemctl reload sshd, then use ssh -vvv: a long pause before SSH2_MSG_SERVICE_ACCEPT is almost always one of these two.