It's also worth noting that we're getting to the point where fastest doesn't really matter. Both Ember and React will be fast enough. Performance shouldn't be a deciding factor.
Performance _does_ enable certain kinds of applications, and it does absolutely matter, even on the desktop.
your concern would be whether to use a framework at all
do you need something to be "at least X(ms) fast" or "faster than what company Y is doing" or "as fast as possible, no exceptions"