I assume this is with back-bdb/hdb and is because Berkeley DB does not scale beyond 8 cores.

From: Daniel Zuniga [mailto:daniel.zuniga@gmail.com]
Sent: Wednesday, July 1, 2020 3:43 PM
To: Quanah Gibson-Mount <quanah@symas.com>
Cc: openldap-technical@openldap.org
Subject: Re: slapd 2.4.44 Performance problems

Is there a reason why OpenLDAP does not seem to use more than 8 cores regardless of the number of threads it is being told to use? With 16 threads it saturates 8 cores, 16 threads and 16 cores still uses 8 cores, 32 threads and 16 cores... only 8 cores are used.