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:
For cached pages things should look better for Drupal 8? Looks like Drupal 8 with PHP 7 is a lot closer to Drupal 7 with PHP 5.6, but still loses by some 20% in delivering raw HTML requests:
So it looks like the promise of speed from Drupal 8 is delivered by peripheral tools such as BigPipe and HTTP Caching, but the raw performance of the Drupal Core itsel seems to have (unsurprisingly) not benefitted from adding layers of abstraction in the form modern components such as Symfony HttpKernel and others.
So especially if you're looking for a performance boost for users who are logged in, it looks like Drupal 8 brings little improvements in it's first iterations. Looks like there'll be plenty of life left in the classic Drupal implementation and the Backdrop CMS based on it.
I have a feeling, that Drupal community is yet to have a real discussion about Drupal 8 being slower and more resource-hungry.