Hi Venu -
just a couple of thoughts regarding your question. Query performance in Tableau against a live data source can be impacted by one or more items: query structure, data structure, dashboard design. If, as you say, the query is executing quickly in native Oracle, then the issue probably resides in one of several other areas - dashboard design, data structure, record volume being processed, network, etc. . What is the performance of the query in Tableau Desktop? Is it similar to native Oracle or Tableau Server? If it's the latter, I would suggest executing a performance recording of the dashboard on both Desktop & Server. This will provide a breakdown of the operations occurring within the dashboard, and hopefully provide insight into the problem area. Info on performance recording can be located here: Create a Performance Recording - Tableau
Some additional thoughts: Are you executing custom SQL, and does it contain sub-queries? Regardless if they've already executed, sub-queries tend to execute each time a filter is modified. Utilization of IsMember user filter is effectively row-level security, and will bypass Cache Server for subsequent use of the dashboard by additional users.