At $dayjob I work on a very dense UI for a financial institution. Think Bloomberg terminal in React and RTK. It's extremely customizable and had lots of elements updating multiple times per second (charts, graphs, enormous tables, etc.) with realtime data coming through over a websocket. We do very little actual performance tuning and mostly just follow best practices.
I don't think React is to blame for your performance issues.
I don't think React is to blame for your performance issues.