We've got our hands on the new iPhone 13 Max Pro and the iPad mini 6. Both feature Apple's 15 Bionic with 6-core CPU and 5-core GPU configuration, but we have confirmed the performance of the chip in the iPad mini is indeed slower on the CPU side, but also on the GPU side too.
https://www.notebookcheck.net/Both-the-CPU-and-GPU-of-the-A15-Bionic-in-the-iPad-mini-confirmed-slower-than-the-iPhone-13-Pro-in-our-early-testing.565003.0.html