For most systems, the default setting of 16 threads is sufficient and provides good import performance. On some systems, increasing the import thread count can lead to improved import performance while selecting a value that is too large can actually cause import performance to degrade.
If minimizing LDIF import time is crucial to your deployment, you must determine the optimal number of import threads for your system, which is dependent on both the underlying system and the data set being imported.
Use the dsconfig command to set the number of import
$ bin/dsconfig set-backend-prop --backend-name userRoot --set import-thread-count:24