If we're talking about peak performance
a11 and Snapdragon 855 level
If it's sustained performance, it depends on the model
The iPhone x's consistent performance is worse than some of the aggressively scheduled 835.
The continuous performance of the iPhone 8p, which basically maintains the peak performance, is also similar to the 855.
If you play the original God this cpu eating game
The original God of this eat large core performance, heat dissipation can be the case, a11 is better than 855, because the a11 cpu does not constitute a bottleneck. It won't drop a frame every few seconds like pre-855 SoCs.
The a11 processor is equivalent to the Snapdragon 845 processor.
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Because these two processors are made of Samsung Chinami, Tuk Tuk run points can reach more than 300,000, but the performance of the former processor is still more powerful than the Qualcomm Snapdragon 845, especially its GPU u performance is more than 50% of the former.
The A11 is equivalent to the Snapdragon 855 processor.