Stuttering video playback on iPhone XS max.

Hi, I have a problem with my iPhone XS max, that is happening quite often when watching videos.

Example: Videos from iTunes Store. Downloaded to the device to eliminate download or streaming issues.

When I playback a movie like "the Martian" on my iPhone, it quite often stutters and fails to deliver proper frame rates.

For sure, this is totally unacceptable for the most expensive consumer smart phone on the market.

My old iPhone 7 plus has no problem maintaining constant playback quality with the same movies.

I observed one thing: When I disable the automatic brightness control of the screen, it seems to work much better. The same happens when I cover the nodge with my thumb.

I have the feeling, that the light sensor delivers some permanently changing values and the system tries to adjust the brightness and video playback permanently, eating up the performance and resulting in the stuttering video. But this is just some guessing.

After several updates of iOS, there is no change in behavior.

Does anybody else have this problem?

Maybe my phone is just defective and I have to replace it.