Views format: faster 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.