Earlier quoted context omitted.
And there is a simple solution to that (poor multi-core utilization): go back to a one-window, one process model (go back, as in delete a lot of unnecessary code); forget the browser tabs - let the OS handle multitasking and let the window manager handle the windows.
This is optimizing for the wrong thing (developer convenience vs. user experience). People like using tabs in their browser more than windows. This also ignores the other important benefit of mparch: sandboxing. The browser needs some elevated access but the page content does not.
(I also don't see how a one-process-per-DOM model prevents any other forms of sandboxing inside the browser. The OS doesn't understand webpages and cannot isolate them from each other - the browser does - so sandboxing pages is the browser's responsibility...)