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.

WebRender doesn't work for me, "Unsupported Vendor" :(

Moto G4, Qualcomm Adreno 405, OpenGL ES 3.2

about:config doesn't work either