Firefox was kinda slow sometimes with too much JS on the page on my old phone but still overall usable, and that was a J3 (replaced just last week), so I'd say that's pretty good. Great and lag-free on my s10+ so far. Not convinced optimization should be a priority over features and addons.
As a tester of Firefox Preview: WebRender has been a godsend for mobile browsing -- I have yet to see it choke!