SSHd is by default configured to perform 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;

  1. Open SSHd config file. 

    /etc/ssh/sshd_config
  2. Look for UseDNS and set the value to no 

    UseDNS no
  3. Restart SSHd service

    How to restart SSH service