People say React is easy to optimize, but many of the React apps I use in practice have crummy 50-100 millisecond response times on basic operations like "click button" or "press key in text field". And a significant portion of the performance difference between beta MDN and old MDN was the ~half second the React code took to run in the beta.

I'm pretty sure front-end developers aren't writing slow code on purpose. The most reasonable explanation I can come up with is that React makes it easier to write reliable UI code but harder to reason about performance -- features like "by default, all of a component's descendants are recreated and diffed with the DOM on any state change" point strongly in this direction.