[Dovecot] Slooow imap copy to "Sent" folders

Hello

Many users here use the "copy to Sent folder" using the IMAP protocol with Dovecot 2.1.15 on a FresBSD 9 server.

Since few days this operation ( copy to sent folder ) became longer than before , it could takes several seconds to complete even the message is very short.

Is there a parameter I could tweak to boost this a bit ? may users complain with reason

thank you