Calculation of Recursive Spawning
$T_\infty$ means you have as many processor as you need (theoretically an infinite number). The strategy to assign each iteration to one processor is base on Divide and Conquer. So, at first, one processor split the number of iterations in two, spawn a new thread with the first half and recursively processes the other half. Then this two processors split each half in two chunks each. And so on until you have one iteration on each chunk, so you can compute it in $iter_\infty (i)$ time. Given that you have an infinite number of processors you can recruit twice the number of processors on each new level of this recursion, so each level can be executed in $O(1)$, but still you need to executed each level after the other. In order to have $n$ iterations (leaf on the recursion tree) you will need $O(\log n)$ levels.