> Folia splits the load for the world into multiple threads, but only on a per-region basis, with each 512x512 being (potentially) on a different thread.

A small correction - Folia regions are not Minecraft regions. They don't necessarily align with the 512x512 region grid, and can take different shapes and sizes depending on what the regionizing algorithm is doing. The term "bubble" is used in some of the documentation and may be a more accurate description.