Fenix is rebuilt from the ground up with modern practices in mind, while the current Firefox has a lot of antiquated code. Firefox Android as it stands is more or less a 'port' of the full version of Firefox, while Fenix's technologies are built with the idea of being on phone hardware from day 1. Firefox Android performance was falling behind Chrome by quite a bit, and Fenix keeps pace with Chrome almost completely now. They've also switched to bottom-screen navigation, so you can tap reload or the URL bar without needing to move your hand on medium-to-large devices. Other than that, the goal is to just make it "Firefox, but with the speed people expect of a modern browser" and I'd say they've delivered already. AFAIK, webrender is not enabled by default yet either, so it has the potential to get better later on as well!