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I’ve just replied to this in another thread coincidentally.

For the time being I’d say best left on.
There seems to be something causing the buffering (which is used when it’s switched off) to use the 1080p format for some people at least.

Where’s with it switched On, it will instantly begin playing in the best possible quality and resolution for your internet speed. Which tends to be 4K, if you’re on an ATV 4K and have a 4K tv of course. Otherwise, doesn’t matter.
 
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