[Dovecot] Sieve not fast enough after upgrade?

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@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?