LDAP Login Slowness Issue

Dear All,
We use Spark 2.7.5 and 2.8.3 with the server 4.4.3.
We have LDAP login Slowness Issue which has started after domain server change. Domain is same but the servers have changed. There was no problem before change. We also have some problems for Chat Group. Some users cannot join the group, some users can join but not visible in the group. Also we see empty user list when we login to Spark.
Is there anyone faced that kind of issues before? How can I diagnose the issue?