An upcoming incremental update to iOS could result in some iPhone models throttling the CPU, in certain situations. The feature (not a bug) is basically a workaround for a degraded battery that might otherwise turn off an iPhone unexpectedly. This feature already exists older iPhone models, and soon will be rolled out to newer models.

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Source: Hot Hardware – Apple’s iPhone XR, iPhone XS Subject To CPU Throttling With This Week’s iOS 13.1 Update