This is a browser benchmark of loading 100 declarative shadow roots, each containing <link rel=stylesheet>s for Bootstrap.css, plus some HTML markup, versus roughly the same thing using <link>s in the <head> and light DOM.
Usage:
npx http-server -p 8080 # or your favorite HTTP server
Then navigate to http://localhost:8080/light-dom.html and compare to http://localhost:8080/shadow-dom.html.