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I recently noticed the SSH login on one of my servers was very slow. The number one reason for this probably is a configuration option in sshd_config. The option is called UseDNS and it causes the server to perform a DNS resolution on all incoming requests.

You may disable it by editing sshd_config

vim /etc/ssh/sshd_config

And edit (or add the following line at the bottom of the script)

UseDNS no

Then restart the SSH daemon with

service ssh restart

And you should notice a major improvement next time you connect to your server over SSH.