For a random subset of users, when they view one image, the app downloads two images (eg webp and jpeg) and makes a note of which one decodes fastest; the goal

For a random subset of users, when they view one image, the app downloads two images (eg webp and jpeg) and makes a note of which one decodes fastest; the goal being that in the future they can just download the fastest-decoding one, saving CPU load (and thus battery usage) for everybody in the future. The argument is that downloading two images when you only need one is “deliberately wasting battery”.