When case-sensitivity is not critical, select the Ignore Case check box in the LDAP User Registry panel in the WebSphere administrative console.
Select Reuse Connections in your LDAP User Registry configuration.
Check to see what caching features your LDAP server provides, and take advantage of them. This action is best suited to LDAP servers that do not change frequently.
Starting with WebSphere Application Server - Express for iSeries Version 5.0.1, you can choose the directory type of IBM_Directory_Server instead of SecureWay if you are using the i5/OS Directory Services product on V5R2 and have upgraded to LDAP 4.1. With LDAP 4.1, i5/OS Directory Services yields improved performance because it is programmed to use the new group membership attributes to optimize group membership searches. However, it is required that authorization is case-insensitive, so also select Ignore Case when you choose the directory type of IBM_Directory_Server. See iSeries LDAP: What's New 
(http://www-1.ibm.com/servers/eserver/iseries/ldap/whatsnew.htm) for information regarding the PTFs that are required for LDAP 4.1.