RESOLUTION
Most probably the reason of slow FTP login is reverse-lookup problem. When people connect your server by FTP, your server tries to do reverse lookup of hostname's IP. In case of problem with DNS settings it can lead to connection timeout.
So, make sure that your server's hostname is resolved to the right IP and this IP can be back-resolved from the Plesk server.
ProFTPd also tries to make reverse lookup for client's IP, it may cause delay if the IP cannot be back-resolved. As workaround, you can disable reverse lookup for ProFTPd at all. To do it, use the following directives in the FTP server configuration file /etc/proftpd.conf ($FTPD_CONF in /etc/psa/psa.conf):
<Global>
IdentLookups Off
....
</Global>....
UseReverseDNS Off