Performance of DAM is overall slow so we upgraded it to 6.5 latest version and still we are facing the issue of slowness. Please suggest some ways to improve the performance and we are also following all the best practices suggested by Adobe.
What does slowness mean to you? It is very subjective...
Capture a series of thread dumps while things are 'slow' and use that as a starting point to better understand WHY it's slow. 
- could be a product issue
- could be an index issue -- eg a corruption, or perhaps heavy usage of apache tika that's performing full-text extraction on some large number of assets in some remote datastore
- could be an implementation issue
From there we can better guide you .. but it could be anything without knowing more specific information.
 -- https://github.com/cqsupport/jstackSeries.sh