If you want to debug a performance issue in postgres, then the answer will be similar: "just use EXPLAIN ANALYZE". You could get out a C debugger and step through the source code, and there are cases where that is appropriate (e.g if you're working on Postgres itself, or you're trying to diagnose a bug in postgres), but if all you want to do is debug your query then you should use the postgres-specific debugging tool (EXPLAIN ANALYZE).

Same for React. If you are working on React itself, or you need diagnose an actual bug in the runtime then you'll want to use the JavaScript debugger and step through. If all you need to do is work out why your component isn't updating then you can use the React-specific tool (the extension) to make your life much easier.

If you really wanted to, you could write your own renderer. there’s already several: react-dom, react-native, react-test-renderer. Also, I believe there’s at least one conference talk where a renderer was live coded