Since TYPO3 4.5.0, prepared queries started to be used in Core for frequent queries. Since the effort to successfully parse SQL queries is high, DBAL caches the result of this lengthy process when prepared queries are issued. Out of the box, DBAL will use the transient memory cache backend of TYPO3 to store this information. This allows queries to be cached for the scope of a single request. If Memcached is configured, it can then cache queries for much longer, thus allowing DBAL to be much more efficient. Caching may be configured within localconf.php:

$GLOBALS['TYPO3_CONF_VARS']['SYS']['caching']['cacheConfigurations']['dbal'] = array(
    'backend' => 'TYPO3\\CMS\\Core\\Cache\\Backend\\MemcachedBackend',
    'options' => array(
        // port is mandatory!
        'servers' => array('localhost:11211', 'otherhost:11211', 'thirdhost:11211'),

You need to have memcached installed as a daemon and also as a PHP extension.