See http://docs.oracle.com/javase/8/docs/technotes/guides/jndi/jndi-ldap.html#pooling

LDAP Connection Pooling is disabled when using a custom ssl socket. The custom
socket was used to accept unsigned/untrusted/expired ssl certs. I removed the
use of the custom socket, and also added the required system property to enable
connection pooling when using SSL, which was missing.