I'm interested in what you think it would take to "[make] a competitive browser again" if not major performance improvements.
I know I sound like an asshole, but it's only because I think Firefox is my last chance for a free web and am disappointed with the route it takes.
I really wish they prove me wrong and in 2-3 years Firefox will be a marvel, re-written from scratch in Rust, but I am afraid that even if that's the case it will be too late.
Good thing that's not what's happening.
None of the major Servo/Quantum features in question, including WebRender, are expected to be faster because they're written in Rust. Rather, they're expected to be faster because they've been designed from the ground up for modern hardware (in the case of Stylo, multicore CPUs; in the case of WR, GPUs).
Additionally Mozilla is not re-writing anything from scratch, instead working toward integrating components from Servo into Firefox.