LDAP authentication - ldap_bind() method is very slow

My application has ldap authentication for users.

When i authenticate its taking very long time to complete about 10 to 15 seconds. If I immediately logout and login again. Its just taking 100ms or something very slow time. After some time when i try login again it again taking 10-15 seconds.

It is very fast when we connect from my personal windows machine, but taking time from our web server machine.

What could be the reason behind this issue ?

Below is my code it succeeds but but take time to complete. If I refresh this page again its instantaneous.

ldap_set_option(NULL, LDAP_OPT_DEBUG_LEVEL, 7);

            //Our Ip address \/
$l = ldap_connect("ldap://"); 
ldap_set_option($l, LDAP_OPT_PROTOCOL_VERSION, 3);
ldap_set_option($l, LDAP_OPT_REFERRALS, 0);

//This takes very long time.
ldap_bind($l, "CN=work,OU=XX-ALL,DC=Example,DC=com", "workPassword");