> my vanilla app loaded in 4 seconds and started rendering after ~2 seconds, while the React/Redux app took 50 seconds to first render

I'm all hands up for vanilla JS development, but I really don't think this drastic load/render time difference is caused by React but rather by the poor coding.

React is tiny, and you can see performance degradation like that only if you don't use it right, don't understand the life-cycle or over-engineer