Pairwise summation in Haskell, take two
Linking the other version of pairwiseSum here Pairwise summation in Haskell
That version is an O(nlogn) algorithm, while this one is linear [ie O(n)]. In fact this version is probably the best you can do in Haskell with lists. It's reminiscient of a fold, which is the natural operation on a list data structure.
The previous version has a bunch of wasted effort as the lists are recursively split. Each time it's split the algorithm has to traverse the whole array. So it's perfectly reasonable that the first version is slower