[Dovecot] Sieve not fast enough after upgrade?

[Nikolay Shopik]

I've upgraded mine Dovecot 1.0.15 to 1.2.4 also moved to new Sieve. I 
have simple sieve rule:

if header :contains "Delivered-To" ["bcc at bcc.inblock.ru"] {
         setflag "\\Seen";
}

It basically mark my own messages which are saved to Sent folder. When I 
was on 1.0.15 I never seen unread count in Thunderbird, but suddenly 
after upgrade I seen unread message in Sent folder just after I've sent 
message. If I open Sent folder it already marked as read.
Me wonder if this is actually Dovecot much faster delivers and reports 
to MUA there is new unread message or Sieve not perform fast enough?