I'm seeing the same thing. I've bought every iPhone EVERY year since the IPhone 3G (yes, I didn't have the original) and every phone EVERY year has demonstrated improved performance on release over the last. This is the FIRST year I've ever seen this kind of issue. My iPhone 12 Pro running right next to iPhone 11 Pro does not maintain frame per second or performance as on the iPhone 11 Pro running the same games and apps. It's like the iPhone 12 Pro shuts down the Firestorm cores almost immediately into playing a game of any kind. My phone never gets hot; it's like all of the intensive apps are only running on the Icestorm cores. I've seen some games start at 60 FPS for maybe a minute and then it's like the high performance cores shut off and the game runs on the low performance cores the rest of the time. I have tried the test when I turn on screen recording and the games do run at 60FPS when that is on. There's something NOT RIGHT in the software (I'm hoping it's software, because if it's hardware Apple is going to have a lot of recalled phones.) I'm not sure how we can be in December when the phone released in October and still have these issues. It's so easy to demonstrate side by side with an older phone. Apple, please give us a response on this issue and that a fix is coming!!

Just an update to my reply earlier about this. I installed 14.3 today on my iPhone and it seems to have corrected the gaming issues!