Hi
What the back-end for your multi source universe
The performance tuning of the reports can be done at various levels such as universe level, report level, database level, and server level.
Analysis the report for which performance is low.
Check the SQL and look any unnecessary joins and Avoid unnecessary joins in the report SQL
Please find the below link for Universe Optimization Techniques, check and apply in your universe.
http://scn.sap.com/community/semantic-layer/blog/2014/05/14/universe-optimization-techniques