You're likely experiencing the major performance improvements that come along with Firefox 57. In a few weeks the general release should feel that fast without some of the trade-offs that come with Nightly. The biggest downside to 57 I'm experiencing is that not all extensions have been updated to the new web extensions framework, so there are some workflows that don't work for me at the moment (LastPass, which is working on a new release, and Stylish, which I'm not sure will update).
Thank you so much to alerting me to this, I was completely unaware. Switching to Stylus now, which has the nice side effect of making me readier for 57.
Stylish has a Chrome version so I'd guess they'll port that to Firefox for 57+. In the meantime there's Stylus, which is a fork of the Chrome version of Stylish:
