>I'll bet that React will be a whole bunch faster than whatever DOM reconciliation library you build up from scraps of vanilla JS

You don't have to bet, you can verify the results yourself. Look here at these benchmarks [0] and compare, Simulacra.js is 22-42% faster than React v15 (depending on whether you also use Redux or MobX). My "scraps of vanilla JS" performs faster! Vanilla without frameworks/libraries is and always will be king of the performance game, though.

[0] http://www.stefankrause.net/js-frameworks-benchmark6/webdriv...