I'm building some views that returns many content types that have different fields, so I'm choosing to display results as "rendered entity" instead of "Fields". I would like to know if there are perfomance reasons to use fields or rendered entity.
If you are able to cache the views output, you should not need to think about that problem. If it is not cachable:
I would say rendered entity will cause more overhead due to rendering each field of the content you selected while with fields you could limit it to the needed fields. If rendered entity and fields equal, I would go for rendered entity with display suite.
For actual performance benchmarking try using apache's benchmarking tool for both solutions.
ab -n100 -c1 http://domain.tld/path/to/view
remember to disable caches while running the tests.
I came across this thought as well when working on increasing performance on a site recently. On that particular site and view, using fields instead of rendered entity cut the time in half.
Rendered entity: 3500ms
This, ofcourse, can be a matter of complexity in regards to the entities. We're also using DS on that site, which might influence the results. In either case, it seems like fields is preferable if there are performance issues. It will be a hassle to maintain several different views with fields instead of maintaining one view mode.