wPrime uses a recursive call of Newton's method for estimating functions, with f(x)=x2-k, where k is the number we're sqrting, until Sgn(f(x)/f'(x)) does not equal that of the previous iteration, starting with an estimation of k/2. It then uses an iterative calling of the estimation method a set amount of times to increase the accuracy of the results. It then confirms that n(k)2=k to ensure the calculation was correct. It repeats this for all numbers from 1 to the requested maximum.
At around 4.2GHz the G.Skill kit is performing as we would expect. At 1800MHz DDR, though, we again see because the CPU speed is quite low and the fact that wPrime does benefit from the raw CPU MHz, it does fall back.