code: public function onBlockToHtmlAfter(Varien_Event_Observer $observer)
      {
          if (!$this->canCollect()) {
              return;
          }
  
          /* @var $block Mage_Core_Block_Abstract */
          $block = $observer->getData('block');
  
          // Don't list blocks from Debug module
          if (!$this->canCaptureBlock($block)) {
              return;
          }
  
          $blockInfo = $this->getRequestInfo()->getBlock($block->getNameInLayout());
          $blockInfo->completeRendering($block);
      }
docs: Listens to core_block_abstract_to_html_after event and computes time spent in block's _toHtml (rendering time).
  
  @param Varien_Event_Observer $observer