dovecot 2 low TCP speed (fetching big mail)

[Wojciech Puchar]

i use dovecot 2 under FreeBSD (dovecot2-2.2.15 compiled from ports).

tried both with kqueue enabled or not.

everything works very fast, EXCEPT fetching big mail.

tried multiple clients (thunderbird on windows, alpine on the same server, 
alpine on other unix server connected by 1Gb/s LAN)  and fetching peaks at 
1MB/s. NO SSL!

could you please point me where to search for a problem.

hints:

writing over IMAP (eg. writing to Sent folder) works at full speed.

Doesn't seem like a problem with unix server, every other service goes at speed 
70-100MB/s both directions (like samba, ftp).

tried to turn off "kqueue" compile time option, no difference.

Please give me a hint at least what to check. Does dovecot use other way to 
sent data over TCP socket than other programs?

thank you.