Slow LMTP

[LaurentG]

  Hello,

using  Debian Sarge with CyrusIMAP MURDER 2.2.8  and MTA Postfix, LMTP 
appears slow.

Here are my conf parameters regarding  mail delivery :

POSTFIX
--> main.cf

initial_destination_concurrency = 50
mailbox_transport = lmtp:unix:/var/run/cyrus/socket/lmtp
lmtp_destination_concurrency_limit = 50

CyrusIMAP
--> Frontends(3)/cyrus.conf

mupdate         cmd="mupdate" listen=3905 prefork=1
lmtp            cmd="lmtpproxyd" listen="/var/run/cyrus/socket/lmtp" prefork=5 maxchild=30

--> Mupdate/cyrus.conf
mupdate         cmd="mupdate -m" listen=3905 prefork=1 maxchild=500

--> Backend(4)/cyrus.conf
lmtp            cmd="lmtpd" listen="lmtp" prefork=1 maxchild=100

With theze parameters,  following phenomenon appears :

LMTP dialog between frontend and backend slow down, with an increasing 
mailq. What happends : few frontend lmtpproxy processes fall in a 
CLOSE_WAIT state on the backend(s) lmtp port. Until the timeout nothing 
happends, the process doesn't work anymore. Whenever I kill one of 
those, a new fresh process starts, working fine.

It appears under charge when several dozens of mails are in the mailq.

Is it symptomatic of a bad parameter or a bad network configuration?
If not, what direction can I go for a better diagnostic?
Is it possible to shorten this timeout in order to let a new process 
take place faster?

If more information needed to illustrate the problem, please tell me.
Thanks for any strategy or parameters to look at.
Laurent