I noticed a long pause when connection to my FTP server running proftp on CentOS.
My FTP client would pause at "Connection established, waiting for welcome message…" for several seconds.

The pause occurs when proftpd is trying to identify the remote username, here is the solution:

1. edit /etc/proftpd.conf
2. Find the <Global></Global> section at the end of the config file
3. Add the following between <Global></Global>: IdentLookups off

So it should look like

<Global>
        IdentLookups off
</Global>

Now restart your ftp server to apply the changes

service proftpd restart