Volume and power button huge delays (iPhone 8 Plus, iOS 14.2, no jailbreak)
iOS 14.3 update solved the issue for me.
It's still a mystery, but at least it doesn't bother me anymore.
UPD: The issue re-appeared a week later alongside with antennae losing signal (both in 3G only and 4G/3G mode). Turns out, this was a faulty hardware.