What you're doing there is positing a "qualitative threshold" of sorts where the anti-hedons from the dust specks cause absolutely zero disutility whatsoever. This can be an acceptable real-world evaluation within loaded subjective context.

However, the problem states that the dust specks have non-zero disutility. This means that they do have some sort of predicted net negative impact somewhere. If that impact is merely to slow down the brain's visual recognition of one word by even 0.03 seconds, in a manner that is directly causal and where the dust speck would have avoided this delay, then over 3^^^3 people that is still more man-hours of work lost than the sum of all lifetimes of all humans on Earth to this day ever. If that is not a tragic loss much more dire than one person being tortured, I don't see what could be. And I'm obviously being generous there with that "0.03 seconds" estimate.

Theoretically, all this accumulated lost time could mean the difference between the extinction or survival of the human race to a pan-galactic super-cataclysmic event, simply by way of throwing us off the particular course of planck-level-exactly-timed course of events that would have allowed us to find a way to survive just barely by a few (total, relatively absolute) seconds too close for comfort.

That last is assuming the deciding agent has the superintelligence power to actually compute this. If calculating from unknown future causal utilities, and the expected utility of a dust speck is still negative non-zero, then it is simple abstraction of the above example and the rational choice is still simply the torture.