[CentOS] Dovecot SLOW with sssd in centos 6

Hello,

I have a big performance problem with a mail server using dovecot and
authenticating users via ldap.
The architecture of the machine is a local ldap and mysql server, they are
used by dovecot for authenticating the mail users.

If i use pam_sss the mail server has about 1/8 - 1/10 the performances it
has if i use the pam_ldap.
Even doing a 'time ls -l' on the mail tree (there are about 3000 ldap
users in that directory) it is very very slow, the first time I try to do
it after a reboot, it takes about 6 minutes with sssd, and about 8 seconds
with ldap.

Since I know sssd is recommended, anyone knows that there is anything in
configuration I can try to increase the speed, avoiding me to use the old
pam_ldap module?

thanks in advance,

Fabio Ferrari