All this fancy animations they've added into Australis are non-essentials and impacting their already super busy single threaded model.
I wonder if removing all the user-initiated animation from the UI would improve perceived responsiveness dramatically, since my browser has no tab animation and a new tab appears instantaneously as far as I can tell.
That said, it's amazing in a somewhat shocking sort of way to see just how many layers of abstraction and lines of code are involved in doing something like a tab animation (I notice the article mentions CSS, Direct3D, Cairo, SVG, and OpenGL), and then realise that in terms of human timescale, it's still extremely fast.
I would argue that they're not non-essentials. They may not be functional essentials, but they certainly add to the experience of running a polished, smooth browser.
You'll be happy to note that I've been loaned out to the Electrolysis team. We're making headway on that front for Desktop, and you can already test it on Nightly[1]. Open up a copy of Nightly, and go to File > Open new e10s window. A new window will open that has its tabs use child processes (you can tell when this is working because the tab's title will be underlined).
We hope to enable this by default on Nightly in the near future.
I'm also curious to know how much of the problems still come from "not nativeness" of the UI (up to some point at least, Mozilla was proud to use some very complex architecture instead of the native controls, I don't know the current status, if I understood more OS-native always has benefits, except for not allowing easy "themes" development).