React's assumption isn't actually true. DOM manipulation is fast - all modern browsers use a dirty-bit system where modifying the DOM is basically a few pointer swaps. Layout, however, is slow, and dirtying the DOM requires at least one layout as soon as control returns from JS. If you intersperse measure and modify calls, it's more.
However, what is true is that:
1. Both JS and the DOM are fast enough that you can lay out most reasonable pages within the threshold of visual perception (~150-200ms).
2. If you need to layout the page during an animation frame, you're hosed anyway. You have a frame budget of 16.67ms and layout will take 100ms+ on mobile.
And so the practical, qualitative conclusion is the same: you can use React for your basic page transformations & rendering, but you need to use CSS transforms and offload the work to the GPU if you want to have a prayer of making animations work on mobile. And React also protects you from completely fucking up performance (which is trivially easy with JQuery), so many real-world pages from inexperienced webdevs experience a very significant speedup.
However, what is true is that:
1. Both JS and the DOM are fast enough that you can lay out most reasonable pages within the threshold of visual perception (~150-200ms).
2. If you need to layout the page during an animation frame, you're hosed anyway. You have a frame budget of 16.67ms and layout will take 100ms+ on mobile.
And so the practical, qualitative conclusion is the same: you can use React for your basic page transformations & rendering, but you need to use CSS transforms and offload the work to the GPU if you want to have a prayer of making animations work on mobile. And React also protects you from completely fucking up performance (which is trivially easy with JQuery), so many real-world pages from inexperienced webdevs experience a very significant speedup.