Angry users sent me a link to discussion of what can cause Proftpd delay during authorization.
So I’ve decided to fix it.
Added UseReverseDNS off to config file — didn’t help.
Added GoogleDNS (8.8.8.8) to resolv.conf — didn’t help.
Added IdentLookups off to proftpd.conf — helped.
Thanks to Ray Dehler.
When IdentLookups is turned on it means that Proftpd is trying to identify the remote username via ident (RFC1413) protocol.