To be clear, Project Quantum is the next phase of architecture, post-Electrolysis. We're also simultaneously working on multiple content processes (which is how Chrome often avoids inter-window jank), but not under the Quantum umbrella.
We think we can do better though, which is where Quantum comes in. The Quantum DOM project is designed to solve exactly the problem you're describing, while using fewer resources and scaling to more tabs. Stay tuned!
https://billmccloskey.wordpress.com/2016/10/27/mozillas-quan...
Yes, that's what makes it the "first stage".
The "leap" part is not Electrolysis; as you note that's just table stakes. The "leap" part is what we can work on now that the Electrolysis first stage, which was very labor-intensive, is done.
Gecko is solving this too, with Electrolysis and Quantum DOM. But because the architectures are so different at the DOM level (direct bindings vs. an XPCOM/direct binding hybrid, tracing vs. reference counting, off main thread layout vs. main thread layout, Rust serialization vs. IPDL, etc.) the Servo solution doesn't directly apply to Gecko. So Gecko and Servo are working on their own solutions mostly independently, rather than sharing code as in most of Quantum.
The trade-off is the crazy amount of resources Chrome uses,even on a multicore machine. 20 process spawned and shit like this can bring down any computer. Chrome resource usage is excessive.
A problem that Chrome has solved years ago. So I think this is not really a leap for the web. Just FireFox catching up a bit.
FireFox is my main browser. The way I deal with it is that I start multiple instances as different users. So they run inside their own process. This way I can have a resource hungry page open in one window (For example a dashboard with realtime data visualization) and still work smoothly in another.