Login with LDAP authentication takes 5 seconds

Hi,

I configured my PostgreSQL 10 DB on Debian 9.2 with LDAP authentication
(simple bind mode). While this basically works, it has the strange effect
that the first login with psql takes around 5 seconds. When I reconnect
within 60 seconds, the login completes immediately.

The LDAP server is behind a firewall. So for a test, in pg_hba.conf I put
the LDAP servers IP address instead of its DNS name (for parameter
ldapserver). Like that, all logins complete immediately. But in general I
prefer specifying the DNS name rather than the IP.

When I checked on the DB machine with the following commands
host my.ldap.server.org
dig my.ldap.server.org
both always returned the host name and IP address of the LDAP server
immediately.

Does anyone of you have an explanation for this, or a hint, where I could
do some further investigation?

Thanks,
Andy