code/Debug/Model/Observer.php
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);
    }
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);
    }
Listens to core_block_abstract_to_html_after event and computes time spent in block's _toHtml (rendering time).

@param Varien_Event_Observer $observer