It does, indeed. Not like constantly recording though. I think they apply some heuristics that keep the camera on in the background (e.g if you open on story camera frequently and on the main screen, but not while at a screen when camera is not accesible). Otherwise the device would become noticeably hot, but I haven't encountered such an issue.

The way that would make sense, for that particular example, is to start the camera up when the user begins the interaction to open it. Like how https://instant.page/ works. If it takes 200ms to get the camera running, do it while the gesture to swipe over to the camera is in progress. It will take more than 200ms for the user to physically move.

iOS has events for gesture start, in progress, finished and cancelled.

Assuming that this happens only when the user is actively using the app, I'd guess that the display would consume dramatically more energy, making the camera's usage of the battery negligible, also compared to the benefit of then having a very responsive camera.

In any case, I do think that providing visual feedback to the user is very important. Like a small picture-in-picture-overlay.

Many flagship apps absolutely destroy battery life. Just something users put up with…

Something users often don't observe even.

The OS should warn about it.