Drupal 8 is a leap forward in terms of architecture. It's a significant overhaul of the rather ancient imperative codebase from 2001 to something that is a mix of that and modern Object Oriented Programming methods familiar to developers using Java or other similar more advanced languages than PHP was fifteen years ago.
In early 2014 there was an interesting article on how to build high performance with PHP applications with ReactPHP. The original article discusses using the Full Stack Symfony Framework. Drupal 8 is built upon the Symfony HttpKernel component and other bits and pieces from that very same team.
Drupal 8 is a great leap forward in terms of architecture for the system. It is also advertised as being the fastest Drupal ever. This may be true when all the caching mechanisms such as Bigpipe rendering are in use, but what about for raw performance?
For logged in users without caching the situation seems dire for Drupal 8, with Drupal 7 on PHP 5.6 delivering pager over twice at the rate of Drupal 8 using the fastest PHP runtime, PHP 7: