SSH takes ages to authorise

[Nik Makepeace <nik@nikmakepeace.org>]

When I connect to my Debian potato machine using ssh (via Putty) protocol 1 
or 2, after I have entered the correct password, it takes ages (about 10 
seconds, although sometimes much more) to complete the authorisation process.

Looking at top while this is happening shows sshd using about 2% for the 
duration.

Using Putty top connect to my other Debian (woody) box results in a near 
instantaneous connection, and using this woody machine to ssh to the potato 
machine results in a near instantaneous connection:

Windows -> potato : slow
Windows -> woody : fast
woody -> potato : fast
potato -> woody : fast

I suspect it might be a DNS lookup thing; my windows machine is masq'd by 
the potato machine.  Perhaps it is trying to do a reverse lookup on 
192.168.1.2?

Any suggestions appreciated - 10 seconds is not so long to wait for a 
login, but it can feel like eternity!

Nik