iOS 14.3 fixed my issue too :-)
Misha van den Bosch
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I'm having also having these issues with a platform game that used to work fine on an iPhone 4 and up. Now, with my iPhone 12 Mini, it's lagging. I think it only happens after I suspended it (e.g. by going to another app and back again). When I then "hard close" the game and restart it, it's ok again.
So yeah, looks like some kind of bug and not exactly a performance issue.