Did you experience the following: you just open a website in a browser and do nothing, but CPU usage of your computer grows up to 100% if look to System Monitor? RAM almost not used, Network almost not used, but CPU usage is near 100%? Then, you just close a tab in a browser, and CPU usage drops twice or even more very fast? This is a true sign that the website has background operations that recalculate the whole DOM of a page all the time in a loop to re-render it.
There are very few reasons to re-render the whole page or root component. In most cases need to re-render only the component, that contains the HTML elements, which should be updated. It's easy to see, how often your components re-render themselves while your website works. Just add "console.log("RERENDER")" to a "render" function of your components and see how often it displays this message in a JS console. If you see something crazy, that this line appears 10 times, for example, but you just click once or even do nothing, then it's time to audit your components to reduce unnecessary renderings. This is an article with some tips on how to do that: https://blog.bitsrc.io/5-ways-to-avoid-react-component-re-renderings-90241e775b8c.
If you know other handy tools either to monitor React components resource usage and/or to optimize performance, post them in comments. It will be very interesting to consider and have a database of these tools and tips.
For now I am happy to add one of the most famous tools to measure website performance. This is LightHouse: https://developers.google.com/web/tools/lighthouse .