Mirrors are insanely computationally expensive because the are scene space reflections which is pretty much a no go with modern rendering techniques.
The interactive mirrors in the game are essentially another viewport which renders the scene from another perspective.
Ray tracing is actually the cheapest way of getting proper scene space reflections today, there's a reason why your FPS drops when you view the mirror.
As far as issues with the model, Cyberpunk used a unified TPP/FPP model, in FPP you have no head it's it's only there for the initial shadow pass which does mean you have a head that is rendered into the shadow buffer but after that your head isn't there.
That's why when you switch to a TPP mode e.g. a bike there can be are pop-in for your head and headgear when you see those assets being streamed and loaded.
The delay is likely a glitch due to your head not having a higher priority than other assets in the game.
You don't see that when hacking cameras because there's enough time for your head to load properly.