React Concurrent Mode
This is a wonderful example of a well engineered solution to the wrong problem.  React is a wonderful view library, and if limited to that use it is remarkably fast and easy to use.<p>But the usual way of building things is to treat React as a total web app framework.  The terminology is very confused: they still call it "rendering", even though it now usually includes data fetching, complex manipulations, event processing, etc.  You just `React.render` once when your page first loads and then child components take care of the rest.  It should really be called `React.start`.<p>> The reason for the stutter is simple: once rendering begins, it can’t be interrupted.<p>Rendering should never <i>need</i> to be interrupted.  If your app is architected to have any rendering operation take long enough that it's perceptible to the user, STOP DOING THAT THING DURING RENDERING.<p>Want a performant web app?  Apply separation of concerns properly, and limit your use of React to the view layer.