When the rumour mill started turning for iPhone 12, everyone got all excited for the prospect of a high refresh rate display. Recent rumours have dashed these hopes and the reason why has now been revealed.

iPad Pro 2020 has one of the best displays around and it is, in no small part, due to the luxurious 120Hz ProMotion display. So, naturally, the expectation was that we could soon see this on iPhone 12. After a few months of hope, several sources said it wouldn’t be happening (including MacRumors).
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https://www.trustedreviews.com/news/huge-new-upgrade-for-iphone-12-ditched-and-5g-is-to-blame-4103051