A user on Reddit has posted potential evidence that Apple could be throttling older iPhones relative to the output battery voltage to prevent them from drawing too much power. Although this is not the first instance of throttling on iPhones, it surely has been a subject of much debate.
https://www.notebookcheck.net/Apple-allegedly-throttling-older-iPhones-having-degraded-batteries.270936.0.html
I really doubt this. Those results are those one gets with a 6S on power saver mode. I'd know, I have one and just performed the test. And with the top bar being exactly the same, he almost certainly edited the photo
I think he did the two tests at the different time. If you do test with that app you can see your history of tests. So he opened the first test result in the history list and took a screenshot. Immediately after he took the first screenshot he opened te second test result and took a screenshot. After doing that he had a screenshot of the first result and a screenshot of the second result (this test was taken at a later moment) with the exactly same topbar and clock.