For me performance is pretty dam good all around, smaller library size or some kind of WASM compilation would be dope. Rendering speeds are already faster than most people notice so maybe less focus on that end of things all around and more on bundle size.

Global state management solutions and event emitting ie. redux, vuex, mobx are my biggest gripe across all frameworks and have led me to explore elm, who I am told does this really well.

Testing on the client side is pretty terrible all around. Jest and enzyme is nice when it works but there are scenarios with react hooks and context providers that leave you diving deep into docs and stack overflow for help. When tests take longer to write than the feature itself it really takes the fun out of everything. In general I’d love client side testing to be as good as backend tests where you can write a test and run it and trust it without needing to jump back to the browser all the time. Exploring cypress and react testing framework along with typescript to alleviate the pain of this has been my only respite.