It is continually surprising to me that there are all these reverse engineered minecraft servers yet none of them seem to provide a significant performance advantage within a single world. Cuberite, Glowstone, etc. They provide a notable boost when your machine is serving many worlds, but it seems most still use a single thread per world model. At least cuberite offloads chunk generation to a second thread, but in their own documentation they claim they don't want to support arbitrary levels of multithreading for fear it would hurt performance on anything less than a quad core. Even though this is a bit of a misconception, I don't understand why they wouldn't make such a thing configurable.