This sounds very interesting even to those of us who don't need a promises library. If you or anyone else would like to go into detail, I bet a lot of people here would care (including me).
I'll second that. Might be appropriate for a series of blog posts, even.
Thanks for your interest - the optimization guide is planned to go over everything I have done.
esailija is the author, and I believe he mentioned that he is planning to write an optimization guide (specific to the library, though it will probably contain generally useful info).