A benchmark on Doctrine 2 and: No caching, caching via useResultCache(true) and caching with explicitly specifying a cache ID.

BasicController.php

<?php
namespace Talis\SwiftForumBundle\Controller;
use Doctrine\ORM\EntityManager;
use Talis\SwiftForumBundle\Controller\BaseController;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Symfony\Component\HttpFoundation\Response;
class BasicController extends BaseController
{
    private $start;
    private $pause_time;

    /*  start the timer  */
    private function start() {
        $this->start = $this->get_time();
        $this->pause_time = 0;
    }

    /*  pause the timer  */
    private function pause() {
        $this->pause_time = $this->get_time();
    }

    /*  unpause the timer  */
    private function unpause() {
        $this->start += ($this->get_time() - $this->pause_time);
        $this->pause_time = 0;
    }

    /*  get the current timer value  */
    private function get_current($decimals = 8) {
        return round(($this->get_time() - $this->start),$decimals);
    }

    /*  format the time in seconds  */
    private function get_time() {
        list($usec,$sec) = explode(' ', microtime());
        return ((float)$usec + (float)$sec);
    }

    /**
     * Return the list of icons in JSON format.
     *
     * @Route("/icons", name="basic_icons")
     */
    public function iconsAction()
    {
        /** @var EntityManager $em */
        $em = $this->getDoctrine()->getManager();
        $iterations = 500;
        $noCaching = 0;
        $setKey = 0;
        $noKey = 0;

        $testNoCaching = 1;
        $testSetKey = 1;
        $testNoKey = 1;

        if($testNoCaching)
        {
            $this->start();
            for($x = 0; $x < $iterations; $x++) {
                $iconsNoCaching = $em->getRepository($this->getNameSpace() . ':Icons')
                    ->getIconsNoCaching();
            }
            $noCaching = $this->get_current();
        }
        
        if($testNoKey)
        {
            $this->start();
            for($x = 0; $x < $iterations; $x++) {
                $iconsNoKey = $em->getRepository($this->getNameSpace() . ':Icons')
                    ->getIconsAutoKey();
            }
            $noKey = $this->get_current();
        }

        if($testSetKey)
        {
            $this->start();
            for($x = 0; $x < $iterations; $x++) {
                if($em->getConfiguration()->getResultCacheImpl()->contains('icons')) {
                    $iconsSetKey = $this->getDoctrine()->getManager()->getConfiguration()->getResultCacheImpl()->fetch('icons');
                } else {
                    $iconsSetKey = $em->getRepository($this->getNameSpace() . ':Icons')
                        ->getIcons();
                }
            }
            $setKey = $this->get_current();
        }
        return new Response('No Caching: ' . $noCaching . '<br>Set Key: ' . $setKey . '<br>No Key: ' . $noKey);
    }
} 

IconsRepository.php

<?php
namespace Talis\SwiftForumBundle\Model;
use Doctrine\ORM\EntityRepository;
class IconsRepository extends EntityRepository
{

    /**
     * Gets the list of icons
     *
     * @return array
     */
    public function getIcons()
    {
        return $this->createQueryBuilder('i')
            ->orderBy('i.id', 'asc')
            ->getQuery()
            ->useResultCache(true, null, 'icons')
            ->getResult();

    }

    /**
     * Gets the list of icons
     *
     * @return array
     */
    public function getIconsNoCaching()
    {
        return $this->createQueryBuilder('i')
            ->orderBy('i.id', 'asc')
            ->getQuery()
            ->useQueryCache(false)
            ->useResultCache(false)
            ->getResult();
    }

    /**
     * Gets the list of icons
     *
     * @return array
     */
    public function getIconsAutoKey()
    {
        return $this->createQueryBuilder('i')
            ->orderBy('i.id', 'asc')
            ->getQuery()
            ->useResultCache(true)
            ->getResult();
    }

}

Results

Automatically generated key - useResultCache(true) - 1000 iterations:
-- Emptied all caches --
Attempt 1: 23.97340798
Attempt 2: 24.97743416
Attempt 3: 31.65670395
Attempt 4: 24.22964978
Attempt 5: 25.81283689

Set key - useResultCache(true, null, 'icons') - 1000 iterations:
-- Emptied all caches --
Attempt 1: 0.71829009
Attempt 2: 0.6626091
Attempt 3: 0.65458798
Attempt 4: 0.65469503
Attempt 5: 0.66013002

500 Iterations:
-- Emptied all caches --
No Caching: 16.3678689
Set Key: 0.41256189
No Key: 14.29288006

No Caching: 14.808321
Set Key: 0.4795301
No Key: 15.10666299

No Caching: 15.28655291
Set Key: 0.34830499
No Key: 13.46338701

No Caching: 13.26075292
Set Key: 0.52053404
No Key: 12.38359118

No Caching: 15.70081902
Set Key: 0.37106895
No Key: 13.44588995

Turning on the general MySQL log shows that everything behaves as expected:
No Caching = every iteration queries the database
Set Key & No Key = query database only the first time