Ask HN: do Reflows and Repaints really cost too much?

is Reflows & Repaints really cost too much to the point that makes me use something that use twice the memory i should use (ex: ReactJS) ? actually sometimes more, imagine implementing the facebook newsfeed using react and flux, react will have a virtual version of the dom in memory, and you should have all the posts objects saved in stores, so that is a lot of memory, sometimes i feel that the JS community didn't find the right way to build apps until now, what do you think?