It would depend on the project. If performance is important, it should indeed by validated, but then it would greatly help if the project also has a performance

It would depend on the project. If performance is important, it should indeed by validated, but then it would greatly help if the project also has a performance test suite that contributors could run locally, and compare their change against the baseline, and a reviewer could then also verify it themselves. What you do with the performance results really depends on the kind of patch they want to commit. Is it an important bug fix? Then I would get that merged first and worry about fixing performance afterwards. And then you have those patches that improve performance in one area at the cost of decreasing it in another. In that case you might have to use your experience in what is more important to decide whether to accept or reject it.