I noticed that ProFTPd took a few seconds to connect. On a LAN, it should be instant.
Sniffing traffic, it seemed that ProFTPd was first making an auth request using the ident protocol before falling back to user/pass authentication. This seems to be the standard behaviour according to RFC’s 912, 931 and 1413
Some google searching turned up a page over at metafilter concerning connection delay.
Adding this to the ProFTPd configuration file disabled the auth checks and made connecting/authorising instant, as it should be.
On Debian, the ProFTPd config file is at