Jeff Geerling has an interesting post to his site showing the results of some of his own testing around the performance of Drupal 8 versus Drupal 7...and that 8 comes out to be slower than 7. He also includes some of the things that the Drupal project is doing to help the situation.

When some people see reports of Drupal 8 being 'dramatically' slower than Drupal 7, they wonder why, and they also use this performance change as ammunition against some of the major architectural changes that were made during Drupal 8's development cycle.

First, I wanted to give some more concrete data behind why Drupal 8 is slower (specifically, what kinds of things does Drupal 8 do that make it take longer per request than Drupal 7 on an otherwise-identical system), and also why this might or might not make any difference in your choice to upgrade to Drupal 8 sooner rather than later.

He shares the results of some of his own benchmarking on a cluster (bramble) of Raspberry Pis for the requests per second on the standard setup for each version. He includes the output from an XHProf profiling run too, showing the large call stack on both sides, not just Drupal 8. He then talks about some of the Drupal 8 updates that are included to help mitigate some of these issues: architecture changes, easier caching, authenticated user handing and slow loading content management.