The mirror and efficient simulation

Until the mirror appeared, the HPMOR universe could be simulated efficiently, at least as far as we knew. Time travel is limited to a six-hour cache; you can't transfigure arbitrary things, and Harry's attempts to use time travel to solve computational problems failed. This is likely to be deliberate.

So, how does the mirror exist? According to the inscription on the back, the mirror shows the actor's coherent extrapolated volition (CEV). Is this possible to compute efficiently from an actor's source code? I would guess not. (Is this right? Is there a hardness proof?)

Here are some possibilities to preserve efficient simulation:

• The mirror is a trick and doesn't do what it says it does
• Perhaps the NPCs are programmed to have known CEVs
• Causality goes in the other direction. Computing the CEV from the source is hard, but constraining an actor to act in a way consistent with their CEV might not be. Perhaps everyone's CEV is set when they first look in the mirror, and then they are constrained to act in accordance with this. (This is somewhat akin to the way of avoiding paradoxes in a quantum universe with time travel, whereby if you go back in time and shoot your grandmother, the bullet diffracts around her.)
• Maybe the previous item is only true of the NPCs. Is the mirror blank at the end of 110 because Harry is a PC (alternatively: simulating him is the purpose of simulation) and computing his CEV is too hard?