I can assure you that there were no performance-related fixes between 0.9.11 and 0.9.12. In fact, there was exactly a one-character change in Sigil's source code between 0.9.11 and 0.9.12 that would have affected Windows users (who were reformatting very specific css). But if you feel 0.9.12 is behaving much better, then who am I to judge?