Connecting to SSH is slow:
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From some systems, it can seem that the connection is very slow when connecting
to the server. This is due to the default behaviour of SSH, which tries to do a
reverse lookup on the incoming IP number. If this number is not resolvable, then
the result is a wait before the login prompt.

To fix:
In the file /etc/ssh/sshd_config, add the following:

UseDNS no
 
This will disable the reverse lookup, thus eliminating the timeout from DNS.

Be aware that the names of the connecting hosts are no longer logged, only the IP.