Why /Drupal\Core\Form\FormBuilder->renderPlaceholderFormAction is taking so long?
Pretty sure that report is bogus. 

That's just an arbitrary method, not a controller. They somehow identify that as the controller for that request but of course that doesn't make sense. That page lists transactions, so complete requests, not single method calls. So some requests take 2s which it incorrectly identifies as this.

Looks like new relic still can't reliably detect the route/controller for a request. Try our new relic module: https://www.drupal.org/project/new_relic_rpm. It reports the route name for the controller, then you'll see what kind of request this actually is.