Click here to watch Building Really Fast Websites with Drupal 8.

Drupal 8 is going to have better front-end performance for anonymous users out of the box: it is now smart enough to no longer load unnecessary JavaScript. It's also (finally!) going to cache all entities efficiently, so that it doesn't waste time rerendering the same content over and over again.

Because less time needs to be spent on generating the HTML, the page will show up faster on visitors' devices — also mobile devices.

However, you of course still want pages to be personalized. Different visitors have different permissions (and hence different contextual links), different interests (and hence have "new" or "updated" indicators in different places), and so on.
In Drupal 8, we've introduced the necessary foundations to easily serve the same HTML (well, >90% the same — i.e. the most expensive parts), yet still personalize the page by using client-side caching (localStorage/sessionStorage) in combination with clever client-side cache invalidation.

Furthermore, we finally have pluggable CSS and JavaScript optimization (aggregation), where you can easily plug in a specific JS minifier, or data-mining-applied-to-visitor-navigation-paths-powered CSS/JS grouping (to calculate globally optimal groups)!

The end result: really fast websites, with superb perceived performance!

Note: where possible, I'll explain how to achieve the same in Drupal 7.

Slides: see wimleers.com/talk/really-fast-drupal-8.