To measure Directory Server performance, you prepare the server, then subject it to the kind of client application traffic you expect in production. The better you reproduce the kind of access patterns client applications that happen in production, the better job you can do sizing the hardware and configuring Directory Server appropriately.
Directory Server Resource Kit provides the authrate(1), modrate(1), and searchrate(1) commands you can use for basic tests. These commands let you measure the rate of binds, modifications, and searches your directory service can support.
You can also simulate, measure, and graph complex, realistic client access using SLAMD. The SLAMD Distributed Load Generation Engine (SLAMD) is a Java application that is designed to stress test and analyze the performance of network-based applications. It was originally developed by Sun Microsystems, Inc. to benchmark and analyze the performance of LDAP Directory Servers. SLAMD is available as an open source application under the Sun Public License, an OSI-approved open source license. To obtain information about SLAMD, go to http://www.slamd.com/. SLAMD is also available as a java.net project. See https://slamd.dev.java.net/.