Apple decided to release the new iPad Air and the iPhone 12 practically simultaneously, a first for the company. The fourth-generation tablet features the same 5 nm chipset as likes of the iPhone 12 and iPhone 12 Pro, so it was inevitable that people would draw performance comparisons. As AnTuTu has demonstrated, it seems that the iPhone 12, at least, is holding back the A14 Bionic somewhat.
https://www.notebookcheck.net/The-iPhone-12-performs-up-to-30-worse-than-the-iPad-Air-with-the-same-Apple-A14-Bionic-chipset-but-that-is-acceptable.498272.0.html