IIS 8.5 time-take for logons is very high

Good day , 

I have 4 Exchange Servers 2013 SP1 installed on Windows Server 2012 R2   , 2 CAS and 2 Mailbox 

I'm having delay problem on CAS Servers , time-taken is high most of the time and sometimes more than 20 seconds 

Users have logs like 2 faild logon records ( SC-Status 401) then 2 successful logon records (SC-Status 200)

The delay is affecting some third party applications functionality , like journalling ,

there is no problem regarding hardware performance 

Authentication methods used are Basic and Windows Authentication ,

virtual directory is RPC

Kindly advice 

Regards,