Go hook up the React dev tools to Reddit and look how much each component renders. You see that components are constantly rendering even though nothing apparent has changed. I get that ideally React would only re-render when something has changed, but clearly it’s easy to create situations where react thinks something has changed even if it hasn’t.
Another good example is the Facebook website itself. It’s just so slow. I get saying Reddit developers suck, but I don’t think you can say the same about FB developers given that FB made React. React is popular because it’s a good developer experience. It’s not the best choice if you want optimal performance.