Slapd running very slow

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.