Why is an even-odd split 'faster' for MergeSort?
Looking at the graphing network on Wikipedia and this page for even-odd merge sort it appears that it has a higher complexity than traditional merge-sort. However, at the cost of more operations, notice how the operations remain independent of one another.

That is to say, where as in merge sort you eventually end up merging 4 lists and then 2 lists, which cannot be done in parallel, even-odd merge sort allows you to operate in parallel until the end of the algorithm.

The Wikipedia article also points out that it is reasonably fast for doing sort in graphics hardware. I suspect this is because where as merge sort your inputs to compare-and-swap depend on which previous element was merged, the inputs to compare-and-swap in even-odd-merge-sort are always the same.

Hope that helps!