Recent updates (if you haven't updated since 0.3.0 or so) have doubled & tripled export speed by a bunch of optimizations, especially metatiling. If you really

Recent updates (if you haven't updated since 0.3.0 or so) have doubled & tripled export speed by a bunch of optimizations, especially metatiling. If you really want to parallelize, you can run the ./index.js command in the source version of TileMill to do exports programmatically.