OP here. In my case, the CPU saved was mostly React updating its virtual DOM. Yes, the browser doesn't do repaints when the tab is in the background. But there'

OP here. In my case, the CPU saved was mostly React updating its virtual DOM. Yes, the browser doesn't do repaints when the tab is in the background. But there's plenty of other work that is done before repaints when using a library like React.