One could safely argue that it is better for the entire current world population to suffer a dust speck each than for someone to get tortured for fifty years,

Would anyone challenge this? Doing the arithmetic, if dust specks cause one second of discomfort, having everyone on Earth get specked would be the equivalent of ~222 person-years of specking. If torture is at least 4.5 times as bad as specking on a per-second basis, even a straight total utilitarian calculation with constant badness per second would favor specks on that scale. That seems like a very, very easy case.