iOS 12.1 adds performance throttling to the iPhone X, iPhone 8 and iPhone 8 Plus
If you're using an iPhone X, iPhone 8 Plus or iPhone 8 and recently updated to iOS 12.1, note that Apple's performance management feature has been enabled for your device.
Apple's support page for the iPhone Battery and Performance reveals that the iPhone 8, iPhone 8 Plus and iPhone X will include the performance management feature starting from iOS 12.1. However, performance management may be less noticeable to the user due to the more advanced hardware and software design.
If you recall, this feature caused an uproar among users of older iPhone models last year. They accused Apple of deliberating slowing down their devices. The performance management feature throttles the CPU performance on the iPhone to prevent it from shutting down unexpectedly.