Performance with OpenLDAP 2.0.26 and Samba

[root@lb-bbs1.emd.ni.schule.de (root)]

I hope this is not too much off topic. I think the coexistence of Samba and
OpenLDAP is subject.
For a school in Germany I set up OpenLDAP with Samba as a PDC.
The Performance seemed to be fine at first.
But now I noticed that about every 90 minutes, when the next lesson begins
and ~160 User log in, the server's CPU gets 100% loaded for periods of
several seconds.
I noticed that slapd gets called for every "net use L: \\myserver\myShare".
I assume that samba binds with the ldap to see if the user has the
appropriate rights (groups etc). That would be 160 Users * 10 Binds.
Has anyone experienced equal problems?
My indices:
index   sn,uid,mail             eq,sub
index   objectClass             pres,eq
index   uidNumber,gidNumber,rid eq

ldbm database definitions:
database        ldbm
cachesize       50000
dbcachesize     4000000
sizelimit       50000
threads         24
dbnosync
dbsync          2 12 5

I need the "mail" index for postfix. All others are more or less guesswork
from me.
The Server has enough RAM (1GB) but CPU is short (1GHz PIII).

Any suggestions? Thanks,
Malte Mueller