Slapd running very slow

[Geoff Swan <gswan3@bigpond.net.au>]

I have an ldap server that appears to slow down significantly even under
moderate load, and wondered if there were any suggestions as to where to
look.

Using openldap-2.4.40 with mdb backend, on a dual Xeon (6 cores each)
server with 128GB ram.
Linux kernel 3.13, separate ext4 db volume (SAS). Slapd configured with
48 threads.

The problem appears that as soon as there are a handful of write
operations, slapd appears to stall. Binding to the ldap port takes many
seconds (instead of instant), and the slapd process appear to occupy
100% of a single CPU core.