You may think using react-native in the browser is not performant, but actually Nicolas Gallagher is a genius. react-native-web is a highly optimized framework that removes any unused modules with a set of complex babel presets and webpack (this is partly what Expo helps to simplify).
There are a number of performance tools at your disposal that will not only optimize your web app, but also improve the performance of your native app!
Expo automatically generates a web-build/report.html with Webpack Bundle Analyzer. A plugin that will help you visualize the size of your static bundles. You can use this to identify abnormally large things that you may not need (like this description).
You can test your project with the Audit tab in Chrome, or with the Lighthouse CLI.
lighthouse <url> --view