Drupal 8 is being developed in the Symfony framework. You may have been exposed to a deployment running in development mode (app_dev.php in the URL is a good sign even if not a necessity).
Once the debug flag is turned off, Symfony won't check the freshness of the dependency container as well as templates, bootstrap caches, etc. (you clear the cache manually in production.)
As to what the Drupal devs may have added on top of the framework, I have no idea. Being a generic CMS, I presume their abstractions are a little bit overengineered.
AFAIK Drupal doesn't use symfony "framework edition" , but symfony/http kernel library at its core to abstract the default PHP http api. So it is a Symfony app as much as Laravel is ... it's not Symfony full stack.
Once the debug flag is turned off, Symfony won't check the freshness of the dependency container as well as templates, bootstrap caches, etc. (you clear the cache manually in production.)
As to what the Drupal devs may have added on top of the framework, I have no idea. Being a generic CMS, I presume their abstractions are a little bit overengineered.