Resolution:
When you click on Query Designer, in background it is issuing a SQL query if you use TIBCO JDBC Driver  

SELECT username AS TABLE_SCHEM, null AS TABLE_CATALOG FROM all_users ORDER BY TABLE_SCHEM

If you have lot of Schemas in DB then it will take a time to run above schema and then it has to go thru each of above TABLE_SCHEM and then display them.

We have seen if you use Oracle JDBC Driver Classes12.zip , then it issues query like :

SELECT username AS table_schem FROM all_users ORDER BY table_schem    

And, that take a shorter time to run and it brings up Query Designer in 10 to 15 seconds.