Performance recording is a nice way to troubleshoot performance issues.
We also use Oracle DB and have found Create a Performance Recording pretty useful .
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1) remove the custom sql and ask the DB team to create materialized view and connect to it
Why because check this link : Connecting Through Custom SQL Causes Slow Performance | Tableau Software
2) If you have any string fields change them to numeric field or boolen
Calcultion like below:
if sum(Sales)>0 then "Green" else "red" end
Change it to:
sum(Sales)>0 it will give true or false conditions use that to evaluate
3) Decrease as much as marks you can
4) Reduce the number of sheets on dashboard by using story points
5)If possible reduce the relative filters
there are many check the below url for the same to know how to increase performance : https://interworks.com/blog/bfair/2015/02/23/tableau-performance-checklist/
Hope this helps , Kindly mark this answer as helpful and correct so that it will help others