Performance improvement

Hi All, Recently we migrated from MySQL to Maria DB, there were views with complex statements are started breaking post migration, We've tried implementing all the known practices (mentioned below) and made them working, however queries are taking quite a long and sometimes not fetching any data (ending up having an error). Could you suggest any improvements that we can made, w.r.t to MariaDb. Due to security issues, I cannot share the script here, but described what we have done.

1. We have implemented CTE (Common table expression) for all the views that are causing issue, this helped improving the performance but not solved the issue 2. Added indexes wherever applicable 3. Tried with both derived_merge on and off

Any other areas we need to focus to improve the performance..

Thanks in Advance