If you observed performance issue in LDAP User Directory, you will have to see if the Directory should be tuned. Even that, if the issue could not be resolved, you may increase the number of connections from Policy Server to the LDAP User Directory so that more requests would be processed.
Please follow the procedures explained in the documentation as following.
Policy Server Configuration Guide 12.52 SP1
User Directories / LDAP Load Balancing and Failover
Configure Load Balancing and Failover
Here is the explanation excerpted from the documentation.
Note: You can add the same server multiple times for load balancing, which forces more requests to a specific system. For example, consider two servers in a group: Server1 and Server2. Server1 is a high-performance server and Server2 is a lesser system. You can add Server1 to the load balancing list twice so that it processes two requests for each request that Server2 processes.