P.S. I haven't looked into this in depth, just throwing an idea out there.
Similar techniques have been used in other types of programs where performance is critical - for instance, user mode networking and user-mode thread scheduling can be quicker than the kernel-mode equivalent because it reduces or eliminates expensive system calls.
From what I understand, the core idea is that changes are applied to the virtual DOM in a very simple way (from the application point of view). Then there is another piece of code that knows how to efficiently find the diff between the vDOM and DOM and apply the changes. It seems that last part could be done by the browser.