I'm not being glib, my current work involves security and privacy, and so I'm more sensitive / concerned about those issues than rendering performance. Current JITs like V8 even give me the willies in terms of attack surface area, such that I'm willing to stomach something like a WASM interpreter for my use cases.
I recognize that WebRender is a huge technique achievement and I absolutely love it when people achieve silky smooth 60fps renders, and if I was working on games or graphics or UX, I'd probably rank that over top of Site Isolation.
The original post mentioned process isolation and flash, so my comment was more a long the lines of 'it's still behind chrome (in isolation)'