Or a hundred such persons?
The calculation itself may be irreducible, but what is the implication for consciousness and agency if, midway through the calculation, you fork the process? What about a quarter of the way, or three quarters?
Whilst this feels unsatisfactory, it is the only way I can see of talking about agency without invoking decision-making processes that disobey the laws of physics, or rely on randomness. And unless a supernatural version of free will appeals, I think this is the best kind of free will available: governed by rules, but nevertheless fundamentally unpredictable. The only other option would be to rely on complete randomness as a basis for decisions, which seems worse.
Just because our perception prevents us from predicting outputs from inputs in a way that appears random does not mean that it is just a chaotic system. This is part of the question posed by the original article: does quantum theory provide certain mechanisms that could permit non-deterministic agents to intervene in causal ways that are not predictable? Or would we have to appeal to the supernatural / mystic / divine in order for such a thing to even be possible?
Certain interpretations (e.g., the "Many Worlds" interpretation of QM) are more conducive to this possibility than others.
Generally, "free will" is simply stated as "the freedom to do otherwise". Returning to the "simulated brain" example -- if the computer simulation is faster than the "squishy brain" agent, then it has already predicted the outcome, and the "squishy brain" agent has no "freedom to do otherwise": the outcome is pre-determined.
> Generally, "free will" is simply stated as "the freedom to do otherwise".
In the Many Worlds interpretation, as I understand it, everything that can happen, does happen (as it would in a single universe of infinite extent). So no matter what you choose to do in one universe, the multiverse-wide outcome is predetermined: copies of you always make all choices not forbidden by the laws of physics.
I think this just pushes the problem of determinism out to the multiverse as a whole.