By default, ssh server performs a reverse DNS lookup for authentication requests. This can be so annoying as you wait ten’s of seconds for the lookup to finish. You may experience a delay before receiving a password prompt while accessing a remote system via ssh.
This guide will show you how to disable SSH reverse DNS lookups in a Linux/Unix system. The method discussed here works for all Linux and Unix based operating systems running OpenSSH Server.
Source: How To Disable SSH reverse DNS Lookups in Linux/Unix system – Computing for Geeks