I will be doing the same. The enhancements in 57 don't out-weight the productivity loss from my critical extensions. I'll be waiting until more of them are ported forward. For the record I don't actually find Firefox 56 slow at the moment either.
I have two critical extensions and a few others that would all break with 56 or 57. No thanks, I'll stick with what works.
Shoving webextensions down everyone's throat, and only afterwards starting to work on implementing APIs and giving anyone a chance to start working on porting stuff. No thanks.
