It is highly recommended to make use of a bytecode cache like APC. A bytecode cache removes the need for parsing PHP code on every request and can greatly improve performance.
As already mentioned earlier in the chapter about configuring Doctrine, it is strongly discouraged to use Doctrine without a Metadata and Query cache (preferably with APC or Memcache as the cache driver). Operating Doctrine without these caches means Doctrine will need to load your mapping information on every single request and has to parse each DQL query on every single request. This is a waste of resources.
Make effective use of the available alternative query result formats like nested array graphs or pure scalar results, especially in scenarios where data is loaded for read-only purposes.
++ Apply Best Practices
A lot of the points mentioned in the Best Practices chapter will also positively affect the performance of Doctrine.
