CodeEvent.php

<?php

public static function fire($event, array $args = array(), $hint = null)
{
  (strpos($event, 'template') === false && (strpos($event, 'load_class') === false || $event === 'load_class') && setHeaderTimer($hint ? $hint : $event));
  ...
}

config.php

<?php

...

function setHeaderTimer($key)
{
  static $counter = 0;
  static $latestKey = null;
  static $latestValue = 0;
  if (!XenForo_Application::debugMode()
    || !XenForo_Application::isRegistered('fc')
    || !XenForo_Application::isRegistered('page_start_time')
  ) {
    return;
  }

  $key = str_replace('XenForo_', '', $key);
  $key = str_replace('_', '-', $key);
  $key = preg_replace('#(.)([A-Z])#', '$1-$2', $key);
  $key = preg_replace('#\-{2,}#', '-', $key);

  $keyBefore = $latestKey;
  $valueBefore = $latestValue;
  $latestKey = sprintf('X-%02d-%s', ++$counter, $key);
  $latestValue = microtime(true) - XenForo_Application::get('page_start_time');
  $valueDelta = $latestValue - $valueBefore;
  if ($valueDelta < .2 && $counter > 1) {
    // return;
  }

  $fcResponse = XenForo_Application::getFc()->getResponse();
  if ($keyBefore !== null) {
    $fcResponse->setHeader($keyBefore, $valueBefore);
  }
  $fcResponse->setHeader($latestKey, sprintf('+%0.6f', $valueDelta));
  $latestKey = null;
}

NOTES.md

X-11-Xenforo-Visitor:1.0679681301117
X-12-Visitor-Setup:3.7609009742737
...
X-20-Xenforo-Datawriter-Attachmentdata:3.9337999820709
X-21-Xenforo-Datawriter-Attachment:5.5158941745758
X-22-Xenforo-Controllerpublic-Attachment:7.168967962265

X-13-Xenforo-Controllerpublic-Attachment:0.039804935455322
X-14-Xenforo-Model-Attachment:1.829607963562

X-13-Xenforo-Controllerpublic-Attachment:0.14207100868225
X-14-Xenforo-Model-Attachment:8.863872051239
...
X-20-Xenforo-Datawriter-Attachmentdata:9.442106962204
X-21-Xenforo-Datawriter-Attachment:11.492645978928

X-11-Xenforo-Visitor:0.48083186149597
X-12-Visitor-Setup:10.060799837112