One of the biggest changes to OpenLDAP in years has made its way into the latest OpenLDAP 2.4 releases, and that is a brand new backend named "back-mdb". This new backend leverages the Lightning Memory-Mapped Database from Symas. To see why this new backend was introduced, it is useful to look at the differences in performance and resource utilization between old BDB based back-hdb and the new LMDB based back-mdb.
Great insight on how much performance improvement you get with OpenLDAP when you use back-mdb instead of back-hdb.