Responsiveness is not the same as performance. Responsiveness is how quickly it responds to user-input. So, for example, if you click on the Back-Button, then it highlights the Back-button to let you know that you have indeed successfully clicked the Back-button and that it is working on going back a page. How quickly it highlights that button after you've clicked it, that's how responsive it is in that aspect. Actually going back a page, i.e. loading the page from cache and rendering it, that's then again a performance-metric.

But you are also quite clearly underestimating how terrible modern browsers are. Mozilla is also currently working on a research browser called "Servo", which has been written completely from scratch and employs a more modern architecture, and that isn't too far away from 400-700% performance increase over other browsers in certain aspects.