iPhone 12 drops frames when recording video

I have an app where I am recording a cropped video (full height, cropped width) at 240 fps. It has been working fine for many years on a wide range of hardware. But when I test on my new iPhone 12 Pro it isn't able to keep up and I am seeing about 10 % dropped frames. In my type of application it is pretty obvious to the user that it is not 100 % OK. It is strange that an iPhone 12 can't run a piece of code that an iPhone 6s handles perfectly (as well as a 7+, an 11Pro and a couple of iPads).

I am using an AVCaptureSession and an AVAssetWriter. I am using "recommendedVideoSettingsForAssetWriterWithOutputFileType: AVFileTypeQuickTimeMovie"
for the asset writer so there shouldn't be any strange format involved.

Is there anything new in the iPhone 12 camera that might cause this? Or have I just gotten a bad copy?
Same here. Other phones such as iPhone 11 can give frames at 240 FPS, but for iPhone 12 we have to drop to 200.
We are also experiencing the same thing. Are there any known workarounds for the issue?
iPhone 12 drops frames when recording video
 
 
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