SSH server normally is configured to perform reverse DNS lookup on every new connetion. Disabling the Reverse Lookup can improve connection speed.

Man sshd_config

UseDNS
Specifies whether sshd(8) should look up the remote host name and check that the resolved host name for the remote IP address maps back to the very same IP address. The default is ‘‘yes’’.

Check

You can observe the problem using tcpdump on port 53 on the ssh server. This situation can slower the sshconnection by 2-4 seconds for every DNS query.

tcpdump port 53

Fix

You can simply modify the ssh config file /etc/ssh/sshd_config

if not present append or modify, if present, the following line:

 useDNS no

restart service

systemctl restart ssh