A major source of performance enhancements in Drupal 8 is from the new system of cache contexts and tags. Normally these are used internally by Drupal to assemble the page from various cached items and not made visible outside the site. In order to debug effective cache clearing, you will need to have the cache tags and context exposed as response headers. Edit your /sites/default/settings.yml file and find the line that has http.response.debug_cacheability_headers: false and change that to true.