Adding to the above issue:
While searching in the logs I found below WARN in tabsvc-tabadmin-java.log file:
"shutting down http_request thread"
Please help to solve the issue.
As good place to start with Tableau Server performance issues would the Tableau Server Performance overview found below.
It gives a break down of some best practices tuning examples as well as additional information on troubleshooting Tableau Server performance.
Hope this helps!