code: public function startRendering(Mage_Core_Block_Abstract $block)
      {
          if ($this->isRendering) {
              // Recursive block instances with same name is used - we don't update render start time
              $this->renderedCount++;
  
              Mage::log("Recursive block rendering {$this->getName()}", Zend_Log::DEBUG);
              return;
          }
  
          // Re-init data from block (some extension might update dynamically block's template)
          $this->init($block);
  
          $this->isRendering = true;
          $this->renderedCount++;
          $this->renderedAt = microtime(true);
  
          if (self::$startRenderingTime===null) {
              self::$startRenderingTime = $this->renderedAt;
          }
      }
docs: Captures rendering start for specified block
  
  @param Mage_Core_Block_Abstract $block
  @throws Exception