The current behaviour of auto_refresh_fast_cache is that it saves on every load. Saving is much slower (at least for memache) than loading, so this should be avoided whenever possible.

I'd prefer cache misses from time to time for the slow backend and loading from slowBackend over always saving to fastBackend.

Possible solution:

Index: /var/www/vpcms/library/zend/1.9.0/Zend/Cache/Backend/TwoLevels.php
===================================================================
--- /var/www/vpcms/library/zend/1.9.0/Zend/Cache/Backend/TwoLevels.php  (revision 29934)
+++ /var/www/vpcms/library/zend/1.9.0/Zend/Cache/Backend/TwoLevels.php  (working copy)
@@ -194,16 +194,18 @@
     public function load($id, $doNotTestCacheValidity = false)
     {
         $res = $this->_fastBackend->load($id, $doNotTestCacheValidity);
+        $loadedFromFast = true;
         if ($res === false) {
             $res = $this->_slowBackend->load($id, $doNotTestCacheValidity);
             if ($res === false) {
                 // there is no cache at all for this id
                 return false;
             }
+            $loadedFromFast = false;
         }
         $array = unserialize($res);
         // maybe, we have to refresh the fast cache ?
-        if ($this->_options['auto_refresh_fast_cache']) {
+        if ($this->_options['auto_refresh_fast_cache'] && !$loadedFromFast) {
             if ($array['priority'] == 10) {
                 // no need to refresh the fast cache with priority = 10
                 return $array['data'];