X(0)) is a smaller value of anti-utility than X(1)), absolutely. I do not, however, know that the decrease of one second is non-negligible for that measurement of anti-utility, under the definitions I have provided.

There are about 1,5 billion seconds in 50 years, so let's define X(n) recursively as torturing ten times more people than in scenario X(n-1) for time equal to 1,499,999,999/1,500,000,000 of time used in scenario X(n-1).

That math gets ugly to try to conceptualize (fractional values of fractional values), but I can appreciate the intention.

since pain is difficult to measure, let's precisely define the way torture is done

This is a non-trivial alteration to the argument, but I will stipulate it for the time being.

At approximately n = 3.8 * 10^10, X(n) means taking 10^(3.8*10^10) people and touching their skin with a hot needle for 1/100 of a second (the tip of the needle which comes into contact with the skin will have 0.0001 square milimeters). Now this is so negligible pain that a dust speck in the eye is clearly worse.

"Clearly"? I suffer from opacity you apparently lack; I cannot distinguish between the two.

Now this seems paradoxical, since going from X(n) to X(n+1) means reducing the amount of suffering of those who already suffer by a tiny amount, roughly one billionth, for the price of adding nine new sufferers for each existing one.

The paradox exists only if suffering is quantified linearly. If it is quantified logarithmically, a one-billionth shift on some position of the logarithmic scale is going to overwhelm the signal of the linearly-multiplicative increasing population of individuals. (Please note that this quantification is on a per-individual basis, which can once quantified be simply added.)

This is far from being a paradox: it is a natural and expected consequence.