SSHd is by default configured to perform reverse DNS Lookup everytime you connect to the server. This is especially true for CentOS/Red Hat and could significantly increase login time.
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”.
To fix this you'll have to disable the UseDNS option on the server via the following steps;
SSHd config file. /etc/ssh/sshd_config
UseDNS and set the value to no UseDNS no