it's worse on newer iPhone models
Keep in mind that newer devices have much faster CPUs, which consume more energy at full speed, so the total energy budget might be the same across models. And this matters because the energy use is what affects standby battery life.
Is there a way to avoid the throttling?
Nothing springs to mind, but I don’t know enough about the specifics of this to give you a definitive answer.
If not is there a reliable way to extend the process time in order to finish the processing?
The only way to extend background execution time is via a
UIApplicationbackground task . See my UIApplication Background Task Notes post for details. You should be able to get about 30 seconds using this API, which sounds like it’s enough to complete your task.
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