Most time is spent normalizing, encoding, decoding, and sorting so-called border states, compact 18*3-bit descriptions of all information needed about a partial

Most time is spent normalizing, encoding, decoding, and sorting so-called border states, compact 18*3-bit descriptions of all information needed about a partial board position to determine its legal full-board completions. This code is fairly optimized, but it's possible that a small factor could be gained by expert analysis. Even so, there's little to no room for reducing the amount of disk-IO, so the end-result might be that the problem becomes more IO bound than CPU bound.