Right now, this is just an educated guess but apple had a problem with 14.5.1 where the system was throttling. Were as I don't have proof, more than likely, the slowness was directly linked to the CPU not running at the peak frequency. I am going to assume here that maybe when apple "Fixed" the performance bug, there are still problems with the CPUs frequency step system. It might be ramping up more than it should.
If so, that would be a direct reason for both the heat, and the decreased battery performance that so many users have been complaining about since 14.5+.
Fingers crossed that if not sooner, that iOS 15 will put an end to this crap! haha