Surely there's a trade off, as we add more and more features to browsers, w3C specifications, add ons, JavaScript updates etc, a browser will inevitably get slower. Yes older browsers were fast, but they did little in comparison to today's virtual machines.
The fact that you can do this https://m.youtube.com/watch?v=BV32Cs_CMqo in a browser should tell you we've come a long way from them just being Document readers.